Refactoring: perform /Att[A-Z']/ -> Attachment$1 renaming

[limonte]

Another question: I noticed there was Att -> Attachment renaming recently.

There are still some /Att[A-Z']/ occurrences and I'm using Mime.createAttNode()

I guess we should also do /Att[A-Z']/ -> Attachment$1, if so I'd prefer it to be done in separate PR not to mix the fix and the refactoring in one PR.

Originally posted by @limonte in #3256 (comment)

[tomholub]

Assignign to @bilalashraf123 to gain some familiarity with the chrome extension.