[securedrop-export] [test + docs] Which files do we support?

[sssoleileraaa]

Description

We need to do another round of printer testing for different file formats that we currently support, and clearly document it. Also, nice-to-have would be a more automated workflow for printer testing.

Here are the test results for Brother and for LaserJet Pro that we've done in the past.

Background

We have a print workflow that moves files from the securedrop-client to a disposable VM that has printer drivers installed. Currently, the team only has access to an HP LaserJet Pro M404n for testing (this lives in my closet most of the time), but in the past we used to have a Brother printer that we also tested against that we are no longer recommending. For more information about the printer workflow, see the original print and export workflow tracking issue

[sssoleileraaa]

Also see https://drive.google.com/drive/u/0/folders/1lyk2V15e7amI9OxvgUK_YhYIUVNDWJo0 for different types of file submissions we can to test for printing/ open in vm (mostly pulled from https://file-examples.com/).