Hi Sammybridge,
not sure, whether this is helpful for you, but have you thought about replacing the standard uploadprocess by doing it via the web api? I personally haven't tried that one but the doumentation says, that you can upload the file via api: https://docs.microsoft.com/en-us/powerapps/maker/portals/web-api-perform-operations . So may be a fileselector and the html5 extension for accessing the file could do the job.
Another (well, a little bit dumb) solution could be to add a field (hidden by css) where you put the name of the uploaded file as a json structure together with the current stepname and a plugin (well, i am old...) could change the name of the attachment (and mark the change as done in the json...).
I have no other idea to find out, when the specific attachment was uploaded in order to relate it to a specific step.
If you use webforms, there is a table caring for the specific steps for a specific entity. You could use that stepid also in order to correlate the upload(s) but i have no idea how to find out, that a specific document was uploaded in a specific stepid.
So, no real solution just some weired ideas.
Hope it helps a little bit.
Have fun,
Christian