Use GM_xmlhttpRequest to POST data on Chrome?
It might be that FormData and multipart/form-data are not well supported on those platforms. Need to look into it more (later).
Meanwhile, try the more typical approach; use application/x-www-form-urlencoded or JSON.
EG:
GM_xmlhttpRequest ( { method: "POST", url: "http://myserver.org/mysscript.py", data: "image64=" + encodeURIComponent (img64), headers: { "Content-Type": "application/x-www-form-urlencoded" }, onload: function (response) { console.log ("gut response"); $("#input").get()[0].value=response.responseText; }} );