handle base64 filename and media type constraints

[wandmagic]

Committer Notes

introduce two constraints for //backmatter/resource/base64 node
check for filename and file type

Making this a draft for now so we can discuss if we would like to verify that we have a valid media type, but it works for just assuring that something is there.

All Submissions:

• Have you selected the correct base branch per Contributing guidance?
• Have you set "Allow edits and access to secrets by maintainers"?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you squashed any non-relevant commits and commit messages? [instructions]
• Have you added an explanation of what your changes do and why you'd like us to include them?
• If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated.?
• If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?

By submitting a pull request, you are agreeing to provide this contribution under the CC0 1.0 Universal public domain dedication.

[aj-stein-gsa]

I'd ask you to hold off on this one, the spreadsheet will not have a lot of detail, so I would read the comment here and the linked issue beneath it.

#644 (comment)

I don't think we have clear answers to a better requirement here or I would have kept it going and asked for more review.

[aj-stein-gsa]

I keep forgetting this one has been put on pause, I am going to close it so I do not keep forgetting and coming back to review it. Sorry for the mixup, @wandmagic.