Calling SteamRemoteStorage.FileShare always gives the result k_EResultFileNotFound

[scottyboy805]

Unity: 2021.3.12
Steamworks.Net: 20.2.0 (But I also tried 20.1.0 initially then updated)
Backend: .Net Standard 2.1 on Mono

I am trying to create a steam leaderboard and attach a replay file to use as a ghost and to verify best times. The steam docs say that I need to use FileShare API to upload the file and then later attach it to the leaderboard entry which all sounds easy enough in theory.

The problem I am having though is that I cannot get the file to upload and the FileShare API always results in the k_EResultFileNotFound error status no matter what I try. Of course I have checked about 100 times that the file exists, tried every path combination I can think of removing anything resembling a special character, replacing backslashes with forward, just file names, no extensions, etc. Still no joy though.

At that point I decided to simplify and switched to the Steamworks.Net test script to rule out anything in my code, but still even this test script results in the same file not found error (Of course I created the required message.dat file beforehand).

At this point I am stuck and think there must either be something incorrect with the steam dashboard setup (I tried enabling ISteamUGC for file transfer just out of curiosity but still nothing) or a bug in Steamworks.Net. I have been over and over the steam docs and I can't find any more information about steps that might need to come before an upload, and the test script linked reasured me that there does not seem to be anymore steps required.

Anyone else had this issue or know anything about it? Any help is much appreciated as I have been at this for hours facing the same error message over and over again, it's enough to drive you mad.