InlineEdit: Add GET /changes/{id}/edit/{path}/type endpoint

Change-Id: I3b5bb2fab294e5ba560ac2db32dcc80055bdc6a8
Add REST API to retrieve file's content type from revision

Change-Id: I0435893cc7cdc0a69f88bb61efccb34593333e1a
Merge changes from topic 'inline-3'

* changes:
  Add REST API to retrieve file's content type from revision
  InlineEdit: Add GET /changes/{id}/edit/{path}/type endpoint