Add getMediaContentType to MediaContent class

Upon looking at the "flickering images bug" (Bug #3165) I noticed that some images from the Guardian feed were not typed correctly. Also, the UI had some own (and different!) code trying to determine the type of media content. Consolidated these into the getMediaContentType method in MediaContent. What do you think? (and yes, I imagine we should probably add more "known file extensions" in there, but I just added .jpeg and .jpg for now).