More consistent typing for Attachments

[sakgoyal]

makes sure you don't accidentally mix types for URL and embedded attachments by removing the ? in the keys

[scsmith]

Hey @sakgoyal Although I agree in theory with this pull request the original content is actually partially generated from the API documentation. I think we need to discuss whether we'd be happy diverging here. Give us a little while to discuss things.

[sakgoyal]

I have a way to make this work, but its not clean. you can keep your existing process, but add these changes to a different branch. then anytime there is an update, just merge these changes onto main if that makes sense

or you could see if there is a way to add a post-processing step (or even a git pre commit hook lol) after generating from the api docs. I think its pretty important to have this change in the library.

But you guys may have different views. I understand if this isnt something you want to change in your processes. (also, this package hasnt been updated in 3 years so will this PR even disrupt that much?)