How to upload a gpx-file from my windows pc to Sandcastle?

Reference:
https://groups.google.com/g/cesium-dev/c/EoU_7AcyJJw
I think this does not work.
Is there a standard procedure or a tool as like “Cesium FTP” to upload own files to Sandcastle ?

Reason:
I want to try out this:
GPX - Cesium Sandcastle
but upload my own GPX file.

Thx in advance for your help
Josef