diff --git a/phpBB/includes/acp/acp_attachments.php b/phpBB/includes/acp/acp_attachments.php
index ebfff442a2..6d7a97022c 100644
--- a/phpBB/includes/acp/acp_attachments.php
+++ b/phpBB/includes/acp/acp_attachments.php
@@ -15,6 +15,7 @@
 * @ignore
 */
 
+use phpbb\attachment\attachment_category;
 use phpbb\attachment\manager;
 use phpbb\config\config;
 use phpbb\controller\helper;
@@ -163,7 +164,7 @@ class acp_attachments
 				}
 				$db->sql_freeresult($result);
 
-				$l_legend_cat_images = $user->lang['SETTINGS_CAT_IMAGES'] . ' [' . $user->lang['ASSIGNED_GROUP'] . ': ' . ((!empty($s_assigned_groups[ATTACHMENT_CATEGORY_IMAGE])) ? implode($user->lang['COMMA_SEPARATOR'], $s_assigned_groups[ATTACHMENT_CATEGORY_IMAGE]) : $user->lang['NO_EXT_GROUP']) . ']';
+				$l_legend_cat_images = $user->lang['SETTINGS_CAT_IMAGES'] . ' [' . $user->lang['ASSIGNED_GROUP'] . ': ' . ((!empty($s_assigned_groups[attachment_category::IMAGE])) ? implode($user->lang['COMMA_SEPARATOR'], $s_assigned_groups[attachment_category::IMAGE]) : $user->lang['NO_EXT_GROUP']) . ']';
 
 				$display_vars = array(
 					'title'	=> 'ACP_ATTACHMENT_SETTINGS',
@@ -584,7 +585,7 @@ class acp_attachments
 
 						$group_ary = array(
 							'group_name'	=> $group_name,
-							'cat_id'		=> $request->variable('special_category', ATTACHMENT_CATEGORY_NONE),
+							'cat_id'		=> $request->variable('special_category', attachment_category::NONE),
 							'allow_group'	=> ($allow_group) ? 1 : 0,
 							'upload_icon'	=> ($upload_icon == 'no_image') ? '' : $upload_icon,
 							'max_filesize'	=> $max_filesize,
@@ -634,10 +635,10 @@ class acp_attachments
 				}
 
 				$cat_lang = array(
-					ATTACHMENT_CATEGORY_NONE		=> $user->lang['NO_FILE_CAT'],
-					ATTACHMENT_CATEGORY_IMAGE		=> $user->lang['CAT_IMAGES'],
-					ATTACHMENT_CATEGORY_AUDIO		=> $user->lang('CAT_AUDIO_FILES'),
-					ATTACHMENT_CATEGORY_VIDEO		=> $user->lang('CAT_VIDEO_FILES'),
+					attachment_category::NONE		=> $user->lang['NO_FILE_CAT'],
+					attachment_category::IMAGE		=> $user->lang['CAT_IMAGES'],
+					attachment_category::AUDIO		=> $user->lang('CAT_AUDIO_FILES'),
+					attachment_category::VIDEO		=> $user->lang('CAT_VIDEO_FILES'),
 				);
 
 				$group_id = $request->variable('g', 0);
@@ -1268,8 +1269,8 @@ class acp_attachments
 
 					$row['extension'] = strtolower(trim((string) $row['extension']));
 					$comment = ($row['attach_comment'] && !$row['in_message']) ? str_replace(array("\n", "\r"), array('<br />', "\n"), $row['attach_comment']) : '';
-					$display_cat = isset($extensions[$row['extension']]['display_cat']) ? $extensions[$row['extension']]['display_cat'] : ATTACHMENT_CATEGORY_NONE;
-					$l_downloaded_viewed = ($display_cat == ATTACHMENT_CATEGORY_NONE) ? 'DOWNLOAD_COUNTS' : 'VIEWED_COUNTS';
+					$display_cat = isset($extensions[$row['extension']]['display_cat']) ? $extensions[$row['extension']]['display_cat'] : attachment_category::NONE;
+					$l_downloaded_viewed = ($display_cat == attachment_category::NONE) ? 'DOWNLOAD_COUNTS' : 'VIEWED_COUNTS';
 
 					$template->assign_block_vars('attachments', array(
 						'ATTACHMENT_POSTER'	=> get_username_string('full', (int) $row['poster_id'], (string) $row['username'], (string) $row['user_colour'], (string) $row['username']),
@@ -1410,10 +1411,10 @@ class acp_attachments
 		global $db, $user;
 
 		$types = array(
-			ATTACHMENT_CATEGORY_NONE		=> $user->lang['NO_FILE_CAT'],
-			ATTACHMENT_CATEGORY_IMAGE		=> $user->lang['CAT_IMAGES'],
-			ATTACHMENT_CATEGORY_AUDIO		=> $user->lang('CAT_AUDIO_FILES'),
-			ATTACHMENT_CATEGORY_VIDEO		=> $user->lang('CAT_VIDEO_FILES'),
+			attachment_category::NONE		=> $user->lang['NO_FILE_CAT'],
+			attachment_category::IMAGE		=> $user->lang['CAT_IMAGES'],
+			attachment_category::AUDIO		=> $user->lang('CAT_AUDIO_FILES'),
+			attachment_category::VIDEO		=> $user->lang('CAT_VIDEO_FILES'),
 		);
 
 		if ($group_id)
@@ -1423,13 +1424,13 @@ class acp_attachments
 				WHERE group_id = ' . (int) $group_id;
 			$result = $db->sql_query($sql);
 
-			$cat_type = (!($row = $db->sql_fetchrow($result))) ? ATTACHMENT_CATEGORY_NONE : $row['cat_id'];
+			$cat_type = (!($row = $db->sql_fetchrow($result))) ? attachment_category::NONE : $row['cat_id'];
 
 			$db->sql_freeresult($result);
 		}
 		else
 		{
-			$cat_type = ATTACHMENT_CATEGORY_NONE;
+			$cat_type = attachment_category::NONE;
 		}
 
 		$group_select = '<select name="' . $select_name . '"' . (($key) ? ' id="' . $key . '"' : '') . '>';
diff --git a/phpBB/includes/constants.php b/phpBB/includes/constants.php
index 286a9f2442..a56a583307 100644
--- a/phpBB/includes/constants.php
+++ b/phpBB/includes/constants.php
@@ -158,10 +158,15 @@ define('CONFIRM_POST', 3);
 define('CONFIRM_REPORT', 4);
 
 // Categories - Attachments
+/* @deprecated Replaced by \phpbb\attachment\attachment_category constants in 4.0.0-a1, to be removed in 4.1.0-a1 */
 define('ATTACHMENT_CATEGORY_NONE', 0);
+/* @deprecated Replaced by \phpbb\attachment\attachment_category constants in 4.0.0-a1, to be removed in 4.1.0-a1 */
 define('ATTACHMENT_CATEGORY_IMAGE', 1); // Inline Images
+/* @deprecated Replaced by \phpbb\attachment\attachment_category constants in 4.0.0-a1, to be removed in 4.1.0-a1 */
 define('ATTACHMENT_CATEGORY_THUMB', 4); // Not used within the database, only while displaying posts
+/* @deprecated Replaced by \phpbb\attachment\attachment_category constants in 4.0.0-a1, to be removed in 4.1.0-a1 */
 define('ATTACHMENT_CATEGORY_AUDIO', 7); // Browser-playable audio files
+/* @deprecated Replaced by \phpbb\attachment\attachment_category constants in 4.0.0-a1, to be removed in 4.1.0-a1 */
 define('ATTACHMENT_CATEGORY_VIDEO', 8); // Browser-playable video files
 
 // BBCode UID length
diff --git a/phpBB/includes/functions_content.php b/phpBB/includes/functions_content.php
index 2218931d6b..523fd15ea2 100644
--- a/phpBB/includes/functions_content.php
+++ b/phpBB/includes/functions_content.php
@@ -14,6 +14,9 @@
 /**
 * @ignore
 */
+
+use phpbb\attachment\attachment_category;
+
 if (!defined('IN_PHPBB'))
 {
 	exit;
@@ -1248,11 +1251,11 @@ function parse_attachments($forum_id, &$message, &$attachments, &$update_count_a
 		{
 			$display_cat = $extensions[$attachment['extension']]['display_cat'];
 
-			if ($display_cat == ATTACHMENT_CATEGORY_IMAGE)
+			if ($display_cat == attachment_category::IMAGE)
 			{
 				if ($attachment['thumbnail'])
 				{
-					$display_cat = ATTACHMENT_CATEGORY_THUMB;
+					$display_cat = attachment_category::THUMB;
 				}
 				else
 				{
@@ -1264,25 +1267,25 @@ function parse_attachments($forum_id, &$message, &$attachments, &$update_count_a
 							{
 								$file_info = $storage_attachment->file_info($filename);
 
-								$display_cat = ($file_info->image_width <= $config['img_link_width'] && $file_info->image_height <= $config['img_link_height']) ? ATTACHMENT_CATEGORY_IMAGE : ATTACHMENT_CATEGORY_NONE;
+								$display_cat = ($file_info->image_width <= $config['img_link_width'] && $file_info->image_height <= $config['img_link_height']) ? attachment_category::IMAGE : attachment_category::NONE;
 							}
 							catch (\Exception $e)
 							{
-								$display_cat = ATTACHMENT_CATEGORY_NONE;
+								$display_cat = attachment_category::NONE;
 							}
 						}
 					}
 					else
 					{
-						$display_cat = ATTACHMENT_CATEGORY_NONE;
+						$display_cat = attachment_category::NONE;
 					}
 				}
 			}
 
 			// Make some descisions based on user options being set.
-			if (($display_cat == ATTACHMENT_CATEGORY_IMAGE || $display_cat == ATTACHMENT_CATEGORY_THUMB) && !$user->optionget('viewimg'))
+			if (($display_cat == attachment_category::IMAGE || $display_cat == attachment_category::THUMB) && !$user->optionget('viewimg'))
 			{
-				$display_cat = ATTACHMENT_CATEGORY_NONE;
+				$display_cat = attachment_category::NONE;
 			}
 
 			$download_link = $controller_helper->route('phpbb_storage_attachment', ['file' => (int) $attachment['attach_id']]);
@@ -1291,7 +1294,7 @@ function parse_attachments($forum_id, &$message, &$attachments, &$update_count_a
 			switch ($display_cat)
 			{
 				// Images
-				case ATTACHMENT_CATEGORY_IMAGE:
+				case attachment_category::IMAGE:
 					$inline_link = $controller_helper->route('phpbb_storage_attachment', ['file' => (int) $attachment['attach_id']]);
 
 					$block_array += array(
@@ -1303,7 +1306,7 @@ function parse_attachments($forum_id, &$message, &$attachments, &$update_count_a
 				break;
 
 				// Images, but display Thumbnail
-				case ATTACHMENT_CATEGORY_THUMB:
+				case attachment_category::THUMB:
 					$thumbnail_link = $controller_helper->route('phpbb_storage_attachment', ['file' => (int) $attachment['attach_id'], 't' => 1]);
 
 					$block_array += array(
@@ -1315,7 +1318,7 @@ function parse_attachments($forum_id, &$message, &$attachments, &$update_count_a
 				break;
 
 				// Audio files
-				case ATTACHMENT_CATEGORY_AUDIO:
+				case attachment_category::AUDIO:
 					$block_array += [
 						'S_AUDIO_FILE'			=> true,
 					];
@@ -1324,7 +1327,7 @@ function parse_attachments($forum_id, &$message, &$attachments, &$update_count_a
 				break;
 
 				// Video files
-				case ATTACHMENT_CATEGORY_VIDEO:
+				case attachment_category::VIDEO:
 					$block_array += [
 						'S_VIDEO_FILE'			=> true,
 					];
diff --git a/phpBB/install/convertors/functions_phpbb20.php b/phpBB/install/convertors/functions_phpbb20.php
index ee60fbf96d..ae67fa1458 100644
--- a/phpBB/install/convertors/functions_phpbb20.php
+++ b/phpBB/install/convertors/functions_phpbb20.php
@@ -11,6 +11,8 @@
 *
 */
 
+use phpbb\attachment\attachment_category;
+
 if (!defined('IN_PHPBB'))
 {
 	exit;
@@ -1393,7 +1395,7 @@ function phpbb_attachment_category($cat_id)
 	switch ($cat_id)
 	{
 		case 1:
-			return ATTACHMENT_CATEGORY_IMAGE;
+			return attachment_category::IMAGE;
 		break;
 
 		case 2:
@@ -1401,7 +1403,7 @@ function phpbb_attachment_category($cat_id)
 		break;
 	}
 
-	return ATTACHMENT_CATEGORY_NONE;
+	return attachment_category::NONE;
 }
 
 /**
diff --git a/phpBB/phpbb/attachment/attachment_category.php b/phpBB/phpbb/attachment/attachment_category.php
new file mode 100644
index 0000000000..60db7ca8df
--- /dev/null
+++ b/phpBB/phpbb/attachment/attachment_category.php
@@ -0,0 +1,32 @@
+<?php
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+namespace phpbb\attachment;
+
+abstract class attachment_category
+{
+	/** @var int None category */
+	public const NONE = 0;
+
+	/** @var int Inline images */
+	public const IMAGE = 1;
+
+	/** @var int Not used within the database, only while displaying posts */
+	public const THUMB = 4;
+
+	/** @var int Browser-playable audio files */
+	public const AUDIO = 7;
+
+	/** @var int Browser-playable video files */
+	public const VIDEO = 8;
+}
diff --git a/phpBB/phpbb/attachment/upload.php b/phpBB/phpbb/attachment/upload.php
index b72f43860c..2cdf61e511 100644
--- a/phpBB/phpbb/attachment/upload.php
+++ b/phpBB/phpbb/attachment/upload.php
@@ -131,7 +131,7 @@ class upload
 		}
 
 		// Whether the uploaded file is in the image category
-		$is_image = (isset($this->extensions[$this->file->get('extension')]['display_cat'])) ? $this->extensions[$this->file->get('extension')]['display_cat'] == ATTACHMENT_CATEGORY_IMAGE : false;
+		$is_image = (isset($this->extensions[$this->file->get('extension')]['display_cat'])) ? $this->extensions[$this->file->get('extension')]['display_cat'] == \phpbb\attachment\attachment_category::IMAGE : false;
 
 		if (!$this->auth->acl_get('a_') && !$this->auth->acl_get('m_', $forum_id))
 		{
diff --git a/phpBB/phpbb/console/command/thumbnail/generate.php b/phpBB/phpbb/console/command/thumbnail/generate.php
index 13104c201f..ac70a2d92e 100644
--- a/phpBB/phpbb/console/command/thumbnail/generate.php
+++ b/phpBB/phpbb/console/command/thumbnail/generate.php
@@ -132,7 +132,7 @@ class generate extends \phpbb\console\command\command
 		$thumbnail_created = array();
 		while ($row = $this->db->sql_fetchrow($result))
 		{
-			if (isset($extensions[$row['extension']]['display_cat']) && $extensions[$row['extension']]['display_cat'] == ATTACHMENT_CATEGORY_IMAGE)
+			if (isset($extensions[$row['extension']]['display_cat']) && $extensions[$row['extension']]['display_cat'] == \phpbb\attachment\attachment_category::IMAGE)
 			{
 				$source = $this->phpbb_root_path . $this->config['upload_path'] . '/' . $row['physical_filename'];
 				$destination = $this->phpbb_root_path . $this->config['upload_path'] . '/thumb_' . $row['physical_filename'];
diff --git a/phpBB/phpbb/db/migration/data/v400/add_audio_files_attachment_group.php b/phpBB/phpbb/db/migration/data/v400/add_audio_files_attachment_group.php
index 97387720ab..1081111736 100644
--- a/phpBB/phpbb/db/migration/data/v400/add_audio_files_attachment_group.php
+++ b/phpBB/phpbb/db/migration/data/v400/add_audio_files_attachment_group.php
@@ -13,6 +13,8 @@
 
 namespace phpbb\db\migration\data\v400;
 
+use phpbb\attachment\attachment_category;
+
 class add_audio_files_attachment_group extends \phpbb\db\migration\migration
 {
 	public static function depends_on()
@@ -40,7 +42,7 @@ class add_audio_files_attachment_group extends \phpbb\db\migration\migration
 		{
 			$sql = 'INSERT INTO ' . $this->table_prefix . 'extension_groups ' . $this->db->sql_build_array('INSERT', [
 				'group_name'		=> 'AUDIO_FILES',
-				'cat_id'			=> ATTACHMENT_CATEGORY_AUDIO,
+				'cat_id'			=> attachment_category::AUDIO,
 				'allow_group'		=> 0,
 				'upload_icon'		=> '',
 				'max_filesize'		=> 0,
@@ -51,7 +53,7 @@ class add_audio_files_attachment_group extends \phpbb\db\migration\migration
 		}
 		else
 		{
-			$sql = 'UPDATE ' . $this->table_prefix . 'extension_groups SET cat_id = ' . ATTACHMENT_CATEGORY_AUDIO . '
+			$sql = 'UPDATE ' . $this->table_prefix . 'extension_groups SET cat_id = ' . attachment_category::AUDIO . '
 				WHERE ' . $this->db->sql_build_array('SELECT', ['group_id' => $audio_group_id]);
 			$this->db->sql_query($sql);
 		}
diff --git a/phpBB/phpbb/db/migration/data/v400/add_video_files_attachment_group.php b/phpBB/phpbb/db/migration/data/v400/add_video_files_attachment_group.php
index cca0b244a8..350816450c 100644
--- a/phpBB/phpbb/db/migration/data/v400/add_video_files_attachment_group.php
+++ b/phpBB/phpbb/db/migration/data/v400/add_video_files_attachment_group.php
@@ -13,6 +13,8 @@
 
 namespace phpbb\db\migration\data\v400;
 
+use phpbb\attachment\attachment_category;
+
 class add_video_files_attachment_group extends \phpbb\db\migration\migration
 {
 	public static function depends_on()
@@ -40,7 +42,7 @@ class add_video_files_attachment_group extends \phpbb\db\migration\migration
 		{
 			$sql = 'INSERT INTO ' . $this->table_prefix . 'extension_groups ' . $this->db->sql_build_array('INSERT', [
 				'group_name'		=> 'VIDEO_FILES',
-				'cat_id'			=> ATTACHMENT_CATEGORY_VIDEO,
+				'cat_id'			=> attachment_category::VIDEO,
 				'allow_group'		=> 0,
 				'upload_icon'		=> '',
 				'max_filesize'		=> 0,
@@ -51,7 +53,7 @@ class add_video_files_attachment_group extends \phpbb\db\migration\migration
 		}
 		else
 		{
-			$sql = 'UPDATE ' . $this->table_prefix . 'extension_groups SET cat_id = ' . ATTACHMENT_CATEGORY_VIDEO . '
+			$sql = 'UPDATE ' . $this->table_prefix . 'extension_groups SET cat_id = ' . attachment_category::VIDEO . '
 				WHERE ' . $this->db->sql_build_array('SELECT', ['group_id' => $video_group_id]);
 			$this->db->sql_query($sql);
 		}
diff --git a/phpBB/phpbb/storage/controller/attachment.php b/phpBB/phpbb/storage/controller/attachment.php
index bde0f357d1..8cfae96905 100644
--- a/phpBB/phpbb/storage/controller/attachment.php
+++ b/phpBB/phpbb/storage/controller/attachment.php
@@ -13,6 +13,7 @@
 
 namespace phpbb\storage\controller;
 
+use phpbb\attachment\attachment_category;
 use phpbb\auth\auth;
 use phpbb\cache\service;
 use phpbb\config\config;
@@ -183,9 +184,9 @@ class attachment extends controller
 		{
 			$attachment['physical_filename'] = 'thumb_' . $attachment['physical_filename'];
 		}
-		else if ($display_cat == ATTACHMENT_CATEGORY_NONE && !$attachment['is_orphan'])
+		else if ($display_cat == attachment_category::NONE && !$attachment['is_orphan'])
 		{
-			if (!(($display_cat == ATTACHMENT_CATEGORY_IMAGE || $display_cat == ATTACHMENT_CATEGORY_THUMB) && !$this->user->optionget('viewimg')))
+			if (!(($display_cat == attachment_category::IMAGE || $display_cat == attachment_category::THUMB) && !$this->user->optionget('viewimg')))
 			{
 				// Update download count
 				$this->phpbb_increment_downloads($attachment['attach_id']);
diff --git a/tests/console/thumbnail_test.php b/tests/console/thumbnail_test.php
index 6982c0074a..e16f3d1e71 100644
--- a/tests/console/thumbnail_test.php
+++ b/tests/console/thumbnail_test.php
@@ -11,6 +11,7 @@
 *
 */
 
+use phpbb\attachment\attachment_category;
 use Symfony\Component\Console\Application;
 use Symfony\Component\Console\Tester\CommandTester;
 use phpbb\console\command\thumbnail\generate;
@@ -57,8 +58,8 @@ class phpbb_console_command_thumbnail_test extends phpbb_database_test_case
 
 		$this->cache = $this->createMock('\phpbb\cache\service');
 		$this->cache->expects(self::any())->method('obtain_attach_extensions')->will(self::returnValue(array(
-			'png' => array('display_cat' => ATTACHMENT_CATEGORY_IMAGE),
-			'txt' => array('display_cat' => ATTACHMENT_CATEGORY_NONE),
+			'png' => array('display_cat' => attachment_category::IMAGE),
+			'txt' => array('display_cat' => attachment_category::NONE),
 		)));
 
 		$this->application = new Application();