#1945 - Extend legal_file helper functions.

- Added get_types_by_extension function, which is a merged version of get...types_by_extension functions (similar to get_extensions).
- Added optional extension argument to get...extensions functions similar to get...types_by_extension functions.
- Added unit tests.  Now, every legal_file function has one.
- Restructured helper file a bit, including adding caches.
- Edited server_add, admin_watermarks, uploader, and item model to use new functionality.