#2010 - Revise item::find_by_path to search for jpg-converted items.

- added extra $type argument to item::find_by_path.
- changed item::find_by_path to use $type to detect if we should look for a jpg-converted item or not (e.g. movie thumbs)
- updated file_proxy to remove the need to guess different movie files.
- moved the album thumb detection to item::find_by_path to ensure it knows to look for an exact album match.