Hi @SeanTambling ,
Firstly, what you are talking about is PowerApps Desktop.
Actually, PowerApps Desktop hasn't relased new version for a long time.
So it may not support many functions (especially new functions).
I've made a similar test about downloading attachments and met the same problem like your description.
In website and mobile device, it could work.
However, in PowerApps desktop, it will not work.
The error message is like this, am I right?

I've reported this problem to our product team, if there's any reply, I will leave my message here.
What's more, I suggest you use PowerApps website.
We are mainly developing website PowerApps.
Best regards,