Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 20697)
+++ wp-includes/media.php	(working copy)
@@ -462,7 +462,8 @@
 			return new WP_Error('resize_path_invalid', __( 'Resize path invalid' ));
 	} else {
 		// all other formats are converted to jpg
-		$destfilename = "{$dir}/{$name}-{$suffix}.jpg";
+		if ( 'jpg' != $ext && 'jpeg' != $ext )
+			$destfilename = "{$dir}/{$name}-{$suffix}.jpg";
 		if ( !imagejpeg( $newimage, $destfilename, apply_filters( 'jpeg_quality', $jpeg_quality, 'image_resize' ) ) )
 			return new WP_Error('resize_path_invalid', __( 'Resize path invalid' ));
 	}