diff --git a/phpBB/adm/style/acp_attachments.html b/phpBB/adm/style/acp_attachments.html
index a8d6f92d71..a2cfe8f11e 100644
--- a/phpBB/adm/style/acp_attachments.html
+++ b/phpBB/adm/style/acp_attachments.html
@@ -29,7 +29,7 @@
 
 	<!-- BEGIN upload -->
 		:: {upload.FILE_INFO}<br />
-		<!-- IF upload.S_DENIED --><span class="error">{upload.DENIED}</span><!-- ELSEIF upload.ERROR_MSG --><span class="error">{upload.ERROR_MSG}</span><!-- ELSE --><span class="success">{L_SUCCESSFULLY_UPLOADED}</span><!-- ENDIF -->
+		<!-- IF upload.S_DENIED --><span class="error">{upload.L_DENIED}</span><!-- ELSEIF upload.ERROR_MSG --><span class="error">{upload.ERROR_MSG}</span><!-- ELSE --><span class="success">{L_SUCCESSFULLY_UPLOADED}</span><!-- ENDIF -->
 		<br /><br />
 	<!-- END upload -->
 
diff --git a/phpBB/includes/acp/acp_attachments.php b/phpBB/includes/acp/acp_attachments.php
index b666d0b8d9..6b7d9f7b44 100644
--- a/phpBB/includes/acp/acp_attachments.php
+++ b/phpBB/includes/acp/acp_attachments.php
@@ -1000,29 +1000,45 @@ class acp_attachments
 						$result = $db->sql_query($sql);
 
 						$files_added = $space_taken = 0;
+						$error_msg = '';
+						$upload_row = [];
 						while ($row = $db->sql_fetchrow($result))
 						{
-							$post_row = $post_info[$upload_list[$row['attach_id']]];
+							$upload_row = [
+								'FILE_INFO'	=> $user->lang('UPLOADING_FILE_TO', $row['real_filename'], $upload_list[$row['attach_id']]),
+							];
 
-							$template->assign_block_vars('upload', array(
-								'FILE_INFO'		=> sprintf($user->lang['UPLOADING_FILE_TO'], $row['real_filename'], $post_row['post_id']),
-								'S_DENIED'		=> (!$auth->acl_get('f_attach', $post_row['forum_id'])) ? true : false,
-								'L_DENIED'		=> (!$auth->acl_get('f_attach', $post_row['forum_id'])) ? sprintf($user->lang['UPLOAD_DENIED_FORUM'], $forum_names[$row['forum_id']]) : '')
-							);
+							if (isset($post_info[$upload_list[$row['attach_id']]]))
+							{
+								$post_row = $post_info[$upload_list[$row['attach_id']]];
+								$upload_row = array_merge($upload_row, [
+									'S_DENIED'	=> !$auth->acl_get('f_attach', $post_row['forum_id']),
+									'L_DENIED'	=> !$auth->acl_get('f_attach', $post_row['forum_id']) ? $user->lang('UPLOAD_DENIED_FORUM', $forum_names[$row['forum_id']]) : '',
+								]);
+							}
+							else
+							{
+								$error_msg = $user->lang('UPLOAD_POST_NOT_EXIST', $row['real_filename'], $upload_list[$row['attach_id']]);
+								$upload_row = array_merge($upload_row, [
+									'ERROR_MSG'	=> $error_msg,
+								]);
+							};
 
-							if (!$auth->acl_get('f_attach', $post_row['forum_id']))
+							$template->assign_block_vars('upload', $upload_row);
+
+							if ($error_msg || !$auth->acl_get('f_attach', $post_row['forum_id']))
 							{
 								continue;
 							}
 
 							// Adjust attachment entry
-							$sql_ary = array(
+							$sql_ary = [
 								'in_message'	=> 0,
 								'is_orphan'		=> 0,
 								'poster_id'		=> $post_row['poster_id'],
 								'post_msg_id'	=> $post_row['post_id'],
 								'topic_id'		=> $post_row['topic_id'],
-							);
+							];
 
 							$sql = 'UPDATE ' . ATTACHMENTS_TABLE . '
 								SET ' . $db->sql_build_array('UPDATE', $sql_ary) . '
@@ -1042,7 +1058,7 @@ class acp_attachments
 							$space_taken += $row['filesize'];
 							$files_added++;
 
-							$phpbb_log->add('admin', $user->data['user_id'], $user->ip, 'LOG_ATTACH_FILEUPLOAD', false, array($post_row['post_id'], $row['real_filename']));
+							$phpbb_log->add('admin', $user->data['user_id'], $user->ip, 'LOG_ATTACH_FILEUPLOAD', false, [$post_row['post_id'], $row['real_filename']]);
 						}
 						$db->sql_freeresult($result);
 
@@ -1054,9 +1070,9 @@ class acp_attachments
 					}
 				}
 
-				$template->assign_vars(array(
-					'S_ORPHAN'		=> true)
-				);
+				$template->assign_vars([
+					'S_ORPHAN'		=> true,
+				]);
 
 				$attachments_per_page = (int) $config['topics_per_page'];
 
@@ -1084,15 +1100,15 @@ class acp_attachments
 
 				while ($row = $db->sql_fetchrow($result))
 				{
-					$template->assign_block_vars('orphan', array(
+					$template->assign_block_vars('orphan', [
 						'FILESIZE'			=> get_formatted_filesize($row['filesize']),
 						'FILETIME'			=> $user->format_date($row['filetime']),
 						'REAL_FILENAME'		=> utf8_basename($row['real_filename']),
 						'PHYSICAL_FILENAME'	=> utf8_basename($row['physical_filename']),
 						'ATTACH_ID'			=> $row['attach_id'],
-						'POST_IDS'			=> (!empty($post_ids[$row['attach_id']])) ? $post_ids[$row['attach_id']] : '',
-						'U_FILE'			=> append_sid($phpbb_root_path . 'download/file.' . $phpEx, 'mode=view&amp;id=' . $row['attach_id']))
-					);
+						'POST_ID'			=> (!empty($post_ids[$row['attach_id']])) ? $post_ids[$row['attach_id']] : '',
+						'U_FILE'			=> append_sid($phpbb_root_path . 'download/file.' . $phpEx, 'mode=view&amp;id=' . $row['attach_id']),
+					]);
 				}
 				$db->sql_freeresult($result);
 
@@ -1105,10 +1121,10 @@ class acp_attachments
 					$start
 				);
 
-				$template->assign_vars(array(
+				$template->assign_vars([
 					'TOTAL_FILES'		=> $num_files,
 					'TOTAL_SIZE'		=> get_formatted_filesize($total_size),
-				));
+				]);
 
 			break;
 
diff --git a/phpBB/language/en/acp/attachments.php b/phpBB/language/en/acp/attachments.php
index 5e0332462a..19fc6b4422 100644
--- a/phpBB/language/en/acp/attachments.php
+++ b/phpBB/language/en/acp/attachments.php
@@ -170,4 +170,5 @@ $lang = array_merge($lang, array(
 	'UPLOAD_DIR_EXPLAIN'			=> 'Storage path for attachments. Please note that if you change this directory while already having uploaded attachments you need to manually copy the files to their new location.',
 	'UPLOAD_ICON'					=> 'Upload icon',
 	'UPLOAD_NOT_DIR'				=> 'The upload location you specified does not appear to be a directory.',
+	'UPLOAD_POST_NOT_EXIST'			=> 'File “%1$s” can not be uploaded to post number %2$d as the post does not exist.',
 ));
diff --git a/tests/functional/acp_attachments_test.php b/tests/functional/acp_attachments_test.php
new file mode 100644
index 0000000000..8b1fb2cc6d
--- /dev/null
+++ b/tests/functional/acp_attachments_test.php
@@ -0,0 +1,118 @@
+<?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.
+*
+*/
+
+/**
+ * @group functional
+ */
+class phpbb_functional_acp_attachments_test extends phpbb_functional_test_case
+{
+	private $path;
+
+	protected function setUp(): void
+	{
+		parent::setUp();
+		$this->path = __DIR__ . '/fixtures/files/';
+		$this->add_lang('posting');
+	}
+
+	protected function tearDown(): void
+	{
+		$iterator = new DirectoryIterator(__DIR__ . '/../../phpBB/files/');
+		foreach ($iterator as $fileinfo)
+		{
+			if (
+				$fileinfo->isDot()
+				|| $fileinfo->isDir()
+				|| $fileinfo->getFilename() === 'index.htm'
+				|| $fileinfo->getFilename() === '.htaccess'
+			)
+			{
+				continue;
+			}
+
+			unlink($fileinfo->getPathname());
+		}
+	}
+
+	private function upload_file($filename, $mimetype)
+	{
+		$crawler = self::$client->request(
+			'GET',
+			'posting.php?mode=reply&f=2&t=1&sid=' . $this->sid
+		);
+
+		$file_form_data = array_merge(['add_file' => $this->lang('ADD_FILE')], $this->get_hidden_fields($crawler, 'posting.php?mode=reply&f=2&t=1&sid=' . $this->sid));
+
+		$file = array(
+			'tmp_name' => $this->path . $filename,
+			'name' => $filename,
+			'type' => $mimetype,
+			'size' => filesize($this->path . $filename),
+			'error' => UPLOAD_ERR_OK,
+		);
+
+		$crawler = self::$client->request(
+			'POST',
+			'posting.php?mode=reply&t=1&sid=' . $this->sid,
+			$file_form_data,
+			array('fileupload' => $file)
+		);
+
+		return $crawler;
+	}
+
+	public function test_orphaned_attachments()
+	{
+		$this->login();
+		$this->add_lang(['common', 'acp/common', 'acp/attachments']);
+		$crawler = $this->upload_file('valid.jpg', 'image/jpeg');
+
+		// Ensure there was no error message rendered
+		$this->assertStringNotContainsString('<h2>' . $this->lang('INFORMATION') . '</h2>', $this->get_content());
+
+		// Also the file name should be in the first row of the files table
+		$this->assertEquals('valid.jpg', $crawler->filter('span.file-name > a')->text());
+		
+		$attach_link = $crawler->filter('span.file-name > a')->attr('href');
+		$attach_id = $this->get_parameter_from_link($attach_link, 'id');
+
+		// Set file time older than 3 hours to consider it orphan
+		$sql = 'UPDATE ' . ATTACHMENTS_TABLE . '
+			SET filetime = filetime - ' . 4*60*60 . '
+			WHERE attach_id = ' . (int) $attach_id;
+		$this->db->sql_query($sql);
+
+		$this->admin_login();
+		$crawler = self::request('GET', 'adm/index.php?sid=' . $this->sid . '&i=acp_attachments&mode=orphan');
+		$this->assertContainsLang('ACP_ORPHAN_ATTACHMENTS_EXPLAIN', $this->get_content());
+		$this->assertStringContainsString('valid.jpg', $crawler->filter('tbody a')->text());
+
+		$form = $crawler->selectButton($this->lang('SUBMIT'))->form([
+			"post_id[$attach_id]"	=> 99999, // Random
+		]);
+		$form["add[$attach_id]"]->tick();
+		$crawler = self::submit($form);
+
+		$this->assertContainsLang('UPLOADING_FILES', $this->get_content());
+		$this->assertStringContainsString($this->lang('UPLOADING_FILE_TO', 'valid.jpg', 99999), $this->get_content());
+		$this->assertStringContainsString($this->lang('UPLOAD_POST_NOT_EXIST', 'valid.jpg', 99999), $crawler->filter('span[class="error"]')->text());
+
+		// Delete the file
+		$form = $crawler->selectButton($this->lang('SUBMIT'))->form();
+		$form["delete[$attach_id]"]->tick();
+		$crawler = self::submit($form);
+
+		$this->assertContainsLang('NOTIFY', $crawler->filter('.successbox')->text());
+		$this->assertStringContainsString(strip_tags($this->lang('LOG_ATTACH_ORPHAN_DEL', 'valid.jpg')), $crawler->filter('.successbox > p')->text());
+	}
+}