Issue title: cannot click back on submitted attachment to double check
 Issue body: I often have the following workflow:

1) write an email
2) attach something to it
3) click on the visible attachment in compose box to download it back, double check it's exactly what I wanted to send (and the last version of it, or whatever)

expected behavior: I can download the file back and open it

actual behavior: not possible to do

![image](https://user-images.githubusercontent.com/6306961/128854482-699a4d9b-4cae-4350-9c4c-2a37b4bd6a90.png)

This would be a great UX improvement, assigning @limonte 

submitted by @Veronkeller 
 Comments: 
 Comment 0: I figure, we should show the preview UI where user can just look at it, or download it if they want - if possible.
{'source': 'GithubIssues', 'repo_name_with_owner': 'FlowCrypt/flowcrypt-browser', 'issue_id': 964850627, 'repo_stars': 257.0}