Enable download/decrypt for shared files when preview is disabled
When file upload preview is disabled, a plaintext "aesgcm" link is displayed in chat. It takes copy/pasting and an external tool (omemo-wget) to open the file.
It makes sense to have a download and open (with default desktop settings / xdg-open) for the file when preview is disabled, so the user doesn't need external tools.
Why making good UX around disabled preview is important:
- preview leaks IP metadata to a third-party server, unless a proxy is enabled ; opting out of message preview makes such a leak opt-in
- file previews are a well-known vector of attacks (in general, not specific to gajim) ; opting out of message preview makes such a security risk opt-in