Welcome to the forum @CosmicFlamingo!
How are you exporting the game? Also, you said you tested in Godot engine, which I assume means the Godot editor? You said you tested on an EXE, but does the EXE also have missing assets or it the issue just on gotm.io?
If the issue is only on gotm.io, I would suggest talking to them about it as its likely something on their end or with the upload process. If the issue occurs on the exported EXE as well, then it may be something with the export process from Godot or the export settings that is causing the issue.