Okay, so I was really close! Ended up figuring out the issue from this post.
The problem was indeed that line I highlighted before, but it just needed a tweak and I did not have to go through the Rest API.
This code from Moe’s solution:
Epicor.ServiceModel.Channels.ImplBase<Ice.Contracts.AttachmentSvcContract>.UriPath)
Needed to be changed to this:
Ice.Proxy.BO.AttachmentImpl.UriPath
After that I was able to download files directly from a button I made through a customization in the Dashboard.
Thank you guys for the help! I’m really impressed by all the responsiveness here, seems like a great community!