feat: git-bug should store metadata for comment's file

[MichaelMure]

At least the mime type should be stored (it's detected when uploaded already) so it can be set properly when downloading without having to load everything in memory to detect it. The name of the file might not be a bad idea either.

This metadata can be extended later for other uses.

These metadata should be stored in the AddCommentOp and the CreateOp so a file can be referenced in different message with different metadata. Bug's commits should only reference the blob's hash to carry files during push/pull.