If I remember it correctly then there needs to be some specification regarding content parsing - probably from W3C, but I’m not sure about it. I saw really long specification for HTML / HTML5 document parsing and as it’s related to it (as a part of <input/> specification) it should be described well or at least linked to 3rd-party specification page. It may be also described or linked on browser developer pages (like MDN) in contents of Gecko or WebKit specifications. My last idea are issue trackers with tickets for adding support for it, so there should be a commit(s) which are encoding form to valid request body, so by knowing how it works somebody could write a parser.
In short somebody would need to find up-to-date specification page and update plug body parser related code.
