One of the cool features of the plugin is the ability to take a snapshot of a full browser page and either save the snapshot to disk or upload it to Wesabe (so you can, for example, save the receipt for a web purchase along with that transaction in your account). The snapshot is uploaded to Wesabe via a standard multipart POST, the same way that a file is uploaded via a web form.
Tim was having trouble getting the POST to work with binary data at first, and he had other things to finish, so he wanted to just base-64-encode it and be done with it. I was reluctant to do that, as the size of the upload would be significantly larger (about 137% of the original). Also, Rails didn't automatically decode base-64-encoded file attachments. But Tim had other bugs to fix, so I submitted a patch to Rails to do the base-64 decoding. I was pretty proud of this patch until it was pointed out to me that RFC 2616 specifically disallows the use of Content-Transfer-Encoding in HTTP. Doh. They also realized that it is a colossal waste of bandwidth.
"some string" + binaryData + "another string", as is necessary when putting together a mutipart post, just does not work.
The answer required employing Rube Goldbergian system of input and output streams. The seed of the solution was found on this post, although that didn't explain how to mix in all of the strings needed for the post and MIME envelope. So here it is, in all it's goriness:
Update: Spaces removed from multipart boundary per Gijsbert's suggestion in the comments (thanks!).