Assignment uploads: check file extension on the client side

[Mai]

In the Canvas API, assignments have an allowed_extensions attribute. These extensions should be checked when running the ua command, and an appropriate error message should be displayed instead of going through with the upload.

Interestingly enough, it seems that the API happily accepts and submits uploads of the wrong file type currently, whereas in my experience, the website doesn't. We still shouldn't allow it though.