Fixed image preview in trashbin subdirs

There are two cases of file names passed to `preview.php`.

1) Files from the root of the trashbin look like `/somefile.jpg.d12345`.
2) Files inside directories of the trashbin look like `/somedir.d12345/somefile.jpg`

For case 2) the code was wrong and passing `somefile` to the mime type preview without extension, which caused it to be of type `application/octet-stream` which produced the wrong icon preview.

Please review @schiesbn @karlitschek @DeepDiver1975 @georgehrke