How do I upload a player file here?
You can find where your character saves are with the locations mentioned in this guide…
How to - Move your saves from Steam cloud to Grim Dawn’s default location
…then you will need to zip them up. If you don’t know how to do that just ask. Character saves are small enough that you can upload them directly to the forum or you can use a cloud service such as Google Drive, Dropbox or anything like that and get a link to the file and then post it.
Depends on what you mean by upload. If you’re talking posting it for others to see you can also use grimtools build calc.
To upload your character to https://www.grimtools.com/calc/ click the ‘up arrow’ button in the upper-lefthand corner. If you have cloud saves enabled (enabled by default) your save file will be in \Program Files (x86)\Steam\userdata########\219990\remote\save\main_{character name}\player.gdc
Otherwise it will be in \Documents\My Games\Grim Dawn\save\main_{character name}\player.gdc
Then click on the icon below the arrow and copy the link shown to paste here.