Update mimetype handling for Debian Stretch

[conorsch]

The submission handling logic relies on custom mimetype associations in order to ensure that submissions are never opened in the sd-svs AppVM (where the SecureDrop Client runs), but rather opens in a network-less DispVM. Once inside the DispVM, mimetypes are also important: the proper application must be selected to ensure a positive user experience. For example, we want to ensure that plaintext submission are opened in gedit, not nano or vim.

Use of xdg-open should be sufficient to ensure proper handling of mimetypes, but some additional dependencies may be required:

• gvfs-bin and libgnomevfs2-bin (to ensure xdg-mime operates correctly)
• nautilus should be present in the sd-svs VM (but isn't included by default in the debian-9 template); we can revisit after Convert sd-journalist to sd-proxy #138 is implemented

We also want to ensure that system paths such as /usr/share/applications/mimeapps.list are not vulnerable to clobbering by upstream changes. It's also an option to declare the mimetype associations in ~/.config/mimeapps.list, but using a non-system path will complicate both packaging logic and the use of TemplateVMs (since AppVMs receive their own $HOME by default, not copied from the base TemplateVM).

See the discussion thread in #155 (comment) for reference.

[conorsch]

While testing #160, noticed that JPG images open in a DispVM in Firefox. That's not what we want (although it does display the image). Let's force an appropriate handler for that type.

[eloquence]

The recommended handlers for different MIME types with various caveats can be found here; see #168 for background.

[kushaldas]

/usr/share/applications/mimeapps.list

Just now verified that on Debian Stretch that file does not exists.

How to test?

sudo apt-get install apt-file
sudo apt-file update
sudo apt-file find /usr/share/applications/mimeapps.list

I even checked this:

~$ sudo apt-file find /usr/share/applications/ | grep list
cinnamon-desktop-data: /usr/share/applications/x-cinnamon-mimeapps.list
gnome-session-common: /usr/share/applications/gnome-mimeapps.list
kde-telepathy-contact-list: /usr/share/applications/org.kde.ktpcontactlist.desktop
knowthelist: /usr/share/applications/knowthelist.desktop

@conorsch We are good to go :)

[conorsch]

Great! Now that we have #182 tracking the package implementation, and judging by @kushaldas's findings on the lack of clobbering risk, the research task identified by this ticket is satisfied.