Allowed Media file types in Global Settings

[rickybanister]

For the slated 'global settings' area:

An 'allowed file types' picker for the media section. Instead of turning on file types one-by-one, we should allow all files that make sense by default (zip, doc, ppt, xls, pdf and so forth) then allow a site admin to pick which their site will handle. A multiselect or checkbox list will be fine to accomplish this.