I double checked and Safari (which I use to download) has zero extensions. This works:
- download (extension adds itself somehow)
- rename *.dms file (remove extension)
- chmod +x (this is required)
- run butler
So, the extension is added in error. Using Safari 11.0.3 over macOS Sierra (not the broken OS that's high on something) 10.12.6
I looked on and found this: https://forums.macrumors.com/threads/safari-erroneously-adding-dms-extension-to-...
It's "possible" that this is fixed in High Sierra (or a late regression on Sierra?) but I wasn't trying to be funny here; really had a bad bad time with that (the latest OS, not butler!) notably when combined with Unity.
Didn't stop me from using Butler overall but not a great experience : )
Thanks for your quick reply by the way.