> 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.

Set fp.encoding may raise an error (eg. for a read-only object, or an object implemented in C). You should add a new argument to the constructor.

> Maybe I'm missing something here, but sys.stdin is always
> a TextIOWrapper instance, even if set to binary mode

I mean: you should pass sys.stdin.buffer instead of sys.stdin.