Support more PP image types

Right now we're supporting:
 * bmp
 * png
 * jpg

GraphicsMagick supports a bunch more formats though. We should probably add a couple more (maybe even all if we're comfortable with that.)
All the supported mime types are declared in oae-preview-processor/lib/constants in the `module.exports.TYPES.IMAGE` array. It might be a matter of finding out the correct mimetype and adding those in.

A list of supported formats can be found at http://www.graphicsmagick.org/formats.html