Add UploadFile.__repr__

[samuelcolvin]

Summary

I'm using UploadFiles directly and it's unhelpful that they don't have an informative repr, this adds that repr in the style of dataclasses.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly. - do I need to do this?