Additionally, it would be sufficiant, if the link of the document in the attachement popup would lead to the public link, then nobody must have be logged into Mattermost server with desktop app AND browser at the same time.

Atm, if you click on a file link in the popup, the browser opens, but leads to a session expired message.

This is kind of similar to https://mattermost.uservoice.com/forums/306457-general/suggestions/17367361-history-of-shared-files-in-a-channel

I'll copy my comment:

What our teams are still missing in Mattermost (Desktop app) is a quick way to open a file and/or jump to its containing folder.

The current behavior is cumbersome - we need to:

1. click the download icon
2. specify a location to save the file
3. manually open the above location in file explorer
4. find the file

It also would be very sweet to have Download & Open button. Which downloads attachment to download folder without asking to specify location. (configurable in options)