Looks like a possible RFC 2616 MUST violation. Apache proxy adds Content-Type header while a no-transform directive prohibits that. See attached trace(s) for details and ways to reproduce the violation mentioned above. Test case IDs in the trace link to human-oriented test case description and RFC quotes, if available.
Created attachment 4449 [details] test case trace
This is a consequence of Apache's DefaultType. *** This bug has been marked as a duplicate of 13986 ***