Opening file implies download

[thamma]

When opening a file via "m n", a non-downloaded file should be downloaded and opened automatically. The default directory is then to be used.

[thamma]

This is currently implemented in branch autodownload, but the code is messy. I separate the code around downloading attachments a bit to support non-interactive downloads

[thamma]

Maybe have a prompt ask (configurable) if the file should be downloaded temporarily (to /tmp) or be saved somewhere into the filesystem (ask user where) when the user wants to open a media document.