submissions/attachments: demonstrate current null Content-Type handling

[alxndrsn]

This probably isn't the desired behaviour, but it is the current behaviour.

[lognaturel]

I don't think this codebase has a ton of comments with links to issues, the expectation is generally to use git blame or equivalent to get context. I don't personally feel strongly about consistency on that point but wanted to flag it in case others have thoughts.

[ktuite]

I'm ok with these references to the issues even if it's a little redundant and git blame could show the trail. (Sometimes just referred to by the issue number, like c#1351.) We have a few sprinkled throughout the code base, especially for tests added to demonstrate specific issues rather than being part of a fix or feature.

I don't think this codebase has a ton of comments with links to issues, the expectation is generally to use git blame or equivalent to get context. I don't personally feel strongly about consistency on that point but wanted to flag it in case others have thoughts.