Just created a test case for this problem after a pentest provoked this error on one of my web apps. Then I found this bug report which already has a similar test case attached.

The problem is that read_binary() as the name says reads binary data, but then writes it to a file which may or may not be binary, depending on whether self._binary_file is set, which depends on whether a filename was set via the content-disposition header.

Jakub's patch looks good and works for me. Please merge this!