3D printing using OctoPrint on Linux Ubuntu desktop
I just discovered that OctoPrint only reads .gcode files for printing (apparently, unless you have a slicing software embedded in it), so I installed Slic3r separately, and ran

./slic3r mystlfile.stl


to generate a .gcode file (in the executable directory).

Then I uploaded the .gcode file to OctoPrint and things got moving.