Slic3r: How can I automatically modify the STL filename? When generating STL files with slic3r I would like to automatically prefix the filename with strings such as PLA or PET before uploading to my printer.  How can I specify this in the filament settings?
The filename is under the "Print settings" tab. Under output options, you can change the name template. You can use words like: [layer_height] to make the name dynamic.

The variable you are looking for here is: [filament_preset]
But the full name, with space and everything, is used.

Here is a screenshot:


Here is a link to fuller docs:
https://github.com/slic3r/Slic3r/wiki/FAQ#how-can-i-specify-a-custom-filename-format-for-output-g-code-files

New version of Slic3r

It lets you edit the filename when upload it. You will see what will be upload it and give it any name. But this is a manual process. Just updated my answer for the sake of completeness.