utils.guess_filename fails if the given parameter looks like a file object but has a non-string name attribute

A cherrypy uploaded file behave like a regular file, except that its name attribute is an int and passing it directly to requests fails because of that

(not sure if i should add myself to AUTHORS for this?)
We have a name for this either in this module or in `requests/compat.py` called `builtin_str`. Please use that instead.