Index: wp-admin/includes/file.php
===================================================================
--- wp-admin/includes/file.php	(revision 15490)
+++ wp-admin/includes/file.php	(working copy)
@@ -315,7 +315,7 @@
 			$file['name'] = $proper_filename;
 
 		if ( ( !$type || !$ext ) && !current_user_can( 'unfiltered_upload' ) )
-			return call_user_func($upload_error_handler, $file, __( 'File type does not meet security guidelines. Try another.' ));
+			return call_user_func($upload_error_handler, $file, __( 'This file type is not allowed. Try another one or extend the file type whitelist by using a plugin.' ));
 
 		if ( !$ext )
 			$ext = ltrim(strrchr($file['name'], '.'), '.');
@@ -429,7 +429,7 @@
 			$file['name'] = $proper_filename;
 
 		if ( ( !$type || !$ext ) && !current_user_can( 'unfiltered_upload' ) )
-			return $upload_error_handler( $file, __( 'File type does not meet security guidelines. Try another.' ));
+			return $upload_error_handler( $file, __( 'This file type is not allowed. Try another one or extend the file type whitelist by using a plugin.' ));
 
 		if ( !$ext )
 			$ext = ltrim(strrchr($file['name'], '.'), '.');