Changed ffmpeg thumbnailer to instead output a full-size png which is forwarded to the image thumbnailer, to allow it to take advantage of all available scaling options
Changed mime type map to deal with the reality that certain file types have multiple extensions and/or multiple mime types, as well as constants supporting all of the data. Created new functions using the updated mime type map to resolve mime types and extensions. Updated various items around the project that determine mime/extension to take advantage of the new functions.
