Handle Galley file type changes

[ajrbyers]

Currently if a Galley's file object is changed when the original file and the new file do not have the same mime this can cause issues. There are two courses of action that can resolve this:

1. When a Galley's file object changes recalculate the type field to ensure it matches the file's mime type
2. If the new type/mime requires XSLT snapshot the current default

Two courses of action have been suggested for this:

1. Add a signal to enact this behaviour
2. Update the Galley object's save method