Comment 24 for bug 1747106

for hpgl files, it is necessary that the extension must be .hpgl, otherwise Inkscape will not recognize this file as being hpgl.

if you rename one of these files to be something like inter.hpgl, then you should get a disclaimer from the Inkscape extension, like the attached file hpgl.png:

if you do not see this disclaimer then it is quite likely that the Inkscape code for hpgl is not being executed. On Windows, the hpgl code is at:

C:\Program Files (x86)\Inkscape\share\extensions\hpgl_input.py