As another data point:
Even though azureai version of Assistant does not officially support the retrieval tool yet, by adding the files to the createMessage makes it work nicely in azureai Assistant. I do not have to refer to the filename in the prompts in either case.
Even though I have it working, I am uneasy that this approach (or any for that matter) is not backed by definitive documentation on what the correct process is. So it might all break on the next push.
On that happy thought
Cheers…