No idea regarding the complexity of this request, but it would be great if you could restrict the file/mime types that are shown when browsing the media library for files.
For example I’ve got a field for clients to upload HTML5 video, so they need to upload MP4 & WebM. So it would be nice to limit the results to these mime-types in this instance.
I’m looking to do this too.. also set a file size limit.
Im thinking the new validate filter in ACF Pro is the answer
http://www.advancedcustomfields.com/resources/acf-validate_value/
I’m going to dig in…