+--- Decode a mime encoded http message body with multipart/form-data
+-- Content-Type. Stores all extracted data associated with its parameter name
+-- in the params table withing the given message object. Multiple parameter
+-- values are stored as tables, ordinary ones as strings.
+-- If an optional file callback function is given then it is feeded with the
+-- file contents chunk by chunk and only the extracted file name is stored
+-- within the params table. The callback function will be called subsequently
+-- with three arguments:
+-- o Table containing the mime headers of the corresponding section
+-- o String value containing a chunk of the file data
+-- o Boolean which indicates wheather the current chunk is the last one (eof)
+-- @param src Ltn12 source function
+-- @param msg HTTP message object
+-- @param filecb File callback function (optional)
+-- @return Value indicating successful operation (not nil means "ok")
+-- @return String containing the error if unsuccessful
+-- @see parse_message_header