Use strongly typed MIME types in Dapps and Signer

[tomusdrw]

Currently we use Strings for mime types, we could consider using https://github.com/hyperium/mime.rs

[rphmeier]

Made some of this change in #2762, but the JS glue codegen needs to generate File instances with a strongly-typed field instead. You have to use the crate directly instead of the hyper::mime module which re-exports it in order to make use of the mime! macro (which is super useful).

[5chdn]

This refactoring labelled issue is inactive since last year.

@paritytech/core-devs Please, confirm within 7 days if this is still realistic to implement some day maybe. If so, please comment below and add an assignee, or close otherwise.

[debris]

is it still relevant? I see that we use mime_guess crate now

[tomusdrw]

We do, but not everywhere. I would close for now or move to long term / easy issues.