GitHub thread posts also use markdown so the same way to post code works there as well.
If it allows attaching the file, on WIndows you can drag and drop.
If GitHub doesn’t like .rb or .skp files, just wrap them in a zip archive.
Ie, at the bottom of the GitHub edit box (below the doted line) you’l see …
"Attach files by dragging & dropping,selecting them, or pasting from the clipboard"
… clicking anywhere on that line will bring up a file browser to select a file.
P.S. - Using the Chrome Web Inspector I can see that this input says … accept=".gif,.jpeg,.jpg,.png,.docx,.gz,.log,.pdf,.pptx,.txt,.xlsx,.zip"
… yes you will need to wrap any .skp files in a zip archive.
But since the code is so simple,… you can just put that in a triple backtick code block.
* There is always a link to the markdown help page at the bottom left of the GitHub issue/thread editbox.