diff --git a/phpBB/includes/functions_upload.php b/phpBB/includes/functions_upload.php
index a0a67ccf3d..ae04fe737c 100644
--- a/phpBB/includes/functions_upload.php
+++ b/phpBB/includes/functions_upload.php
@@ -232,7 +232,12 @@ class filespec
 	{
 		if ($this->mimetype_guesser !== null)
 		{
-			$this->mimetype = $this->mimetype_guesser->guess($filename);
+			$mimetype = $this->mimetype_guesser->guess($filename);
+
+			if (empty($this->mimetype) || $mimetype !== 'application/octet-stream')
+			{
+				$this->mimetype = $mimetype;
+			}
 		}
 
 		return $this->mimetype;
diff --git a/tests/functional/fileupload_form_test.php b/tests/functional/fileupload_form_test.php
index e87953367f..b8c48389e0 100644
--- a/tests/functional/fileupload_form_test.php
+++ b/tests/functional/fileupload_form_test.php
@@ -107,9 +107,9 @@ class phpbb_functional_fileupload_form_test extends phpbb_functional_test_case
 
 		$crawler = $this->upload_file('disallowed.jpg', 'image/jpeg');
 
-		// Hitting the ATTACHED_IMAGE_NOT_IMAGE error means we passed the
+		// Hitting the UNABLE_GET_IMAGE_SIZE error means we passed the
 		// DISALLOWED_CONTENT check
-		$this->assertContains($this->lang('ATTACHED_IMAGE_NOT_IMAGE'), $crawler->text());
+		$this->assertContainsLang('UNABLE_GET_IMAGE_SIZE', $crawler->text());
 	}
 
 	public function test_too_large()