Also: /upload/image allows uploading in two ways: by giving a direct link to the image or by sending the image file itself. I've successfully uploaded images using the first method in Python, but I can't make my script to work with the second one.

FWIW this is possible via /upload/image. Here is another implementation (in Java) apart from the one in Go @rene mentioned. See also this closely related question.

I've added a comment on the PR that introduced the bug.