While I agree that utf8 should be used (whether Slack is involved or not), this is a blame-the-victim solution. Slack is utterly in the wrong here. And who knows what other files they are reading, much less damaging? Are they scanning your uploaded images? Doing ML or "Shazaming" your videos and monetizing the results like Roku and the TV mfrs are? Downscaling your images?

Perhaps they only do it to free accounts, where a benefit of paying is that your files are not molested? That would be both horrifying and yet, at the same time, hilarious.

This is the exact thing that wasn't working and which necessitated me sending various test files with particular encodings to a colleague.

It is seriously troubling that you think the default text encoding for nearly every contemporary text editor WASN'T tried.