Did you ever get an answer to this? I have a similar setup and noticed when switching the rewrite response from 404 (Rewrite) to 200 (Rewrite) I run into this MIME error but when setting it back to 404 (Rewrite) it works.
Reasons:
Blacklisted phrase (1): answer to this?
RegEx Blacklisted phrase (3): Did you ever get an answer to this