Evaluate using `System.Net.Mime` for use with `Constants.MimeTypes`

[dninemfive]

i currently have a dictionary mapping file extensions to MIME types in the Constants class. This can't be replaced wholesale with the native System.Net.Mime.MediaType constants, because several values are missing and they're not mapped to file extensions, but it might be a good idea to replace the string literals for those values which are available in that namespace with references to the constants.

This might require updating to .NET 9.0 to be fully implemented - the Application.GZip constant seems to be missing right now.