Create a mechanism for dynamically determining output of file entities

This relates to File entity and re-usable managed files system #1448

When we have much of the foundation of the managed file system in place, we'll want to make it as easy as possible to show media in the correct way. Images should use the common image formatter, while audio and video should also be able to display correctly automatically.

One approach to this is to have a new file field formatter which dynamically adjust the output based on metadata about the given fid such as mimetype, or file bundle type.

Exactly how this is implemented will need to be determined by the results of the tasks this depends on.