I’ve split your post to a new topic, this isn’t relevant to the feature request you responded to.
So a .zip file? We need more information to really help you. It’s probably a .zip file though. It sounds like first of all you need to find a mail program that can save attachments to other programs! 
Your current one seems rather limited if it can’t save things to Files or Dropbox.
That has nothing to do with Scrivener, naturally. This is one of those weird things that comes along with using iOS. You can’t just save the attachment to the disk somewhere and then open it normally. You have to find programs that can “send” files to each other, and programs that even are capable of unzipping (neither Dropbox nor Files do that I think), etc.
I use “FileApp” for this (not to be confused with Apple’s “Files”). A forum user describes an easy process using this system, in this post.
But again, it sounds like step one is finding a capable mail program. The built-in Mail that Apple provides can save attachments to other programs, so maybe just use that as a fallback when you need it.