#2214: Limit list of allowed file extensions based on file types allowed.
#2214: Limit files that appear in OS file browser by accepted type.

When you click the Upload button, your OS will bring up a browser for your local files.
These files are now filtered by the types the dialog accepts. For instance, media
embed will only let you upload images, photos and videos.
Merge pull request #2571 from openscholar/2214-accurate_help_text

Make help text accurate