Are you setting Content-Type before X-Accel or after? I was just testing X-Accel and ran into the same issue. Turns out I applied Content-Type prior to X-Accel and it worked.

Per the documentation (or a mailing list post somewhere) X-Accel *inherits* Content-Type, so I'd imagine if the server received an X-Accl before Content-Type, the Content-Type would've been "determined" already...

-Michael