submission comment attachment and text

[vahtras]

Hi As I was browsing through the code, I was looking for a way to comment on an assignment with text and an attachment in one step. I was able to do it in two steps with calls to edit() and upload_comment() separately. Have I missed something? If not would you consider a PR for something that appears to be a small fix? I would then be able to do something like this

submission.upload_comment('myfile.txt', text_comment='Here is my file')

to get

Regards, Olav