add getValidMediaTypes method

Adds a method to to SiteMedia to grab all the valid default MediaType's from class.

This allows us to grab a list of valid types (and corresponding mime types and extensions) to use when processing and accepting uploads.

Also adds support for alternative mime types on the MediaType table, which is necessary for certain types of media, such as audio/mp4, which has a large number of mime types we should check against.

For example, WebKit browsers will not allow uploading m4a files, unless the secondary mime type of ```audio/x-m4a``` is added as an acceptable type. The mime type column and extension columns are still used for displaying media.