PostScript files may be viewed with Ghostview, or some other tool that allows panning and zooming within an image. If the image is reduced to fit the screen it will be unreadable.
HPGL files may be viewed with hp2xx, or hpview, or various commercial software eg. dlwin Drawing Librarian. hptoxx should probably use flags -t -q.
In order to view files with an .hpg filetype, it is necessary to obtain an hpgl viewer such as hp2xx, and configure .mailcap and .mime.types appropriately. Ask your local wizard if you don't know how to do this. (Non-Unix systems will have different files, eg. MOSAIC.INI under MS-Windows).