Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 44477)
+++ src/wp-includes/functions.php	(working copy)
@@ -2624,15 +2624,17 @@
 	 * Filters the "real" file type of the given file.
 	 *
 	 * @since 3.0.0
+	 * @since 5.1.0 The $real_mime parameter was added.
 	 *
-	 * @param array  $wp_check_filetype_and_ext File data array containing 'ext', 'type', and
-	 *                                          'proper_filename' keys.
-	 * @param string $file                      Full path to the file.
-	 * @param string $filename                  The name of the file (may differ from $file due to
-	 *                                          $file being in a tmp directory).
-	 * @param array  $mimes                     Key is the file extension with value as the mime type.
+	 * @param array       $wp_check_filetype_and_ext File data array containing 'ext', 'type', and
+	 *                                               'proper_filename' keys.
+	 * @param string      $file                      Full path to the file.
+	 * @param string      $filename                  The name of the file (may differ from $file due to
+	 *                                               $file being in a tmp directory).
+	 * @param array       $mimes                     Key is the file extension with value as the mime type.
+	 * @param string|bool $real_mime                 The actual mime type or false if the type cannot be determined.
 	 */
-	return apply_filters( 'wp_check_filetype_and_ext', compact( 'ext', 'type', 'proper_filename' ), $file, $filename, $mimes );
+	return apply_filters( 'wp_check_filetype_and_ext', compact( 'ext', 'type', 'proper_filename' ), $file, $filename, $mimes, $real_mime );
 }
 
 /**