(defun http-post-encode-multipart-data (fields files coding-system)
  "Return FIELDS and FILES encoded for use as the data for a multipart HTTP POST request"
  (http-post-join-strings
   (mapcar '(lambda (field)
	     (http-post-bound-field
	      (format "Content-Disposition: form-data; name=%S" (symbol-name (car field)))
	      ""
	      (cdr field)))
	   fields)
   (mapcan '(lambda (file)
	     (destructuring-bind (fieldname filename mime-type data) file
	       (http-post-bound-field
		(format "Content-Disposition: form-data; name=%S; filename=%S" fieldname filename)
		(format "Content-type: %s" mime-type)
		""
		data)))
	   files)
   (format "--%s--" (http-post-multipart-boundary))))