Did you test it against server-side form-data parser implementation? It will be useless if it won't work with most widespread implementations: PHP's and at least some others (consider some popular python web frameworks). 

MIME-compliance is not enough, because browsers send only subset of MIME format and server-side parsers don't expect bodies full of MIME features.

Particularly, I believe, most implementations don't expect any "Content-Transfer-Encoding", except "binary", because only 8-bit transfers are implemented in browsers, also you should check there will be no line-splitting and header-folding in headers and content, and make sure CR+LF (not plain LF, which is in patch) is always used.