Content-Type: multipart/mixed; boundary="boundary"

This shouldn't appear.
--boundary
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64

VGhpcyBpc24ndCByZWFsbHkgYW4gYXBwbGljYXRpb24vb2N0ZXQtc3RyZWFtLiA7KQ=='

--boundary
Content-Type: image/png
Content-Transfer-Encoding: base64

TmVpdGhlciBpcyB0aGlzIGFuIGltYWdlL3BuZy4=

--boundary
Content-Type: multipart/related; boundary="boundary2"

--boundary2
Content-Type: text/html

<html><head>This part should be returned.</head></html>

--boundary2--

--boundary
Content-Type: text/plain

This part shouldn't.

--boundary

Neither should this part!

--boundary--
