Inline Edit: Honor project specific MIME types

Projects are allowed to map files to specific MIME types using
configuration settings stored inside of project.config. These
are honored by GetDiff, but were not supported inside of the
two REST APIs that also return content types.

Refactor the content type handling to be consistent between
the APIs that need to return them to the browser.

Change-Id: I1591c5e1ba0618ee2497ab119a088c83a48cb14a
4 files changed