Getting a JSON response instead of a base64 one greatly simplifies the
logic regarding fetching the contents of an edited file.
With this header, 404s no longer occur in valid circumstances -- if the
file is not found, an empty string is used as the file contents.
Bug: Issue 7434
Change-Id: I39042c7c253944195c9dfab84a3b8c5cbe2a1b03