Change mediatypes in the UI to something more user friendly

[emanueldima]

A normal user cannot be expected to know what the mediatype of a Word document is. The list of media types should be changed to a list of easy to understand names.

Other considerations:

• Distinguishing between subtypes of a media type is also needed (e.g. xml can be also tei, which can be also tei-dta).
• Sometimes a set of media types are equivalent, due to historical reasons (e.g. audio/wav, audio/vnd.wave, audio/x-wav)