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Non-animated GIFs embed just fine, if that's what you mean. I just tried animated again, and sure enough, attachment.

S3 bucket, FTP folder, imgur... for your custom gif's and then you can paste a URL and get animation. No frills.

If I paste a URL, I get a URL, not an embedded image. Then people have to click it to see the gif, and they don't, so it defeats the purpose.

That is a preference you disabled. By default URLs are expanded inline.

Copy and paste it into slack instead

Uhh.... that creates an attachment.

I'm begging to suspect that there's a setting, hidden size threshold, or something involved.

Could be a setting in your organization or something I guess. Uploading a gif works fine for me and it's animated.