@Glenn
"Also, the use of FeedParser could be replaced by BytesFeedParser, thus eliminating the need to decode header lines in that loop."

BytesFeedParser only uses the ascii codec ; if the header has non ASCII characters (filename in a multipart/form-data), they are replaced by ? : the original file name is lost. So for the moment I leave the text version of FeedParser

@Victor :
"you should use qs.encode(locale.getpreferredencoding(), 'surrogateescape')"
Ok, I changed the code to that

"Maybe a DeprecationWarning if we would like to drop support of TextIOWrapper later :-)"
Maybe I'm missing something here, but sys.stdin is always a TextIOWrapper instance, even if set to binary mode

"For the else case: you should maybe add a strict test on the type, eg. check for RawIOBase or BufferedIOBase subclass, isinstance(fp, (io.RawIOBase, io.BufferedIOBase)). It would avoid to check that fp.read() returns a bytes object (or get an ugly error later)."

Rejecting non-instances of RawIOBase or BufferedIOBase is too much, I think. Any class whose instances have a read() method that return bytes should be accepted, like the TestReadLine class in test_cgi.py

"Set sys.stdin.buffer.encoding attribute is not a good idea. Why do you modify fp, instead of using a separated attribute on FieldStorage (eg. self.fp_encoding)?"

I set an attribute encoding to self.fp because, for each part of a multipart/form-data, a new instance of FieldStorage is created, and this instance needs to know how to decode bytes. So, either an attribute must be set to one of the arguments of the FieldStorage constructor, and fp comes to mind, or an extra argument has to be passed to this constructor, i.e. the encoding of the original stream