#1945 - Extend legal_file helper functions - STILL NEEDS ADMIN_WATERMARK.PHP
Add get_types_by_extension function, which is to get_..._types_by_extension functions what get_extensions is to the get_..._extensions functions (i.e. a merged version of them).
Add optional extension argument to get_..._extensions functions similar to get_..._types_by_extension functions.
Add new unit test cases.
Edit server_add, admin_watermarks, uploader, and item model to use new functionality.