Sorry that I don’t know the answer to your question, but I’m sure some of the Zapier experts here will know the answer. (Worst case scenario, I believe that Zapier gives you the ability to send your own custom API requests to Airtable, so that would be one way to do it.)
Not sure if you’re fully committed to Zapier or not, but this is one of the reasons that I really like using Integromat for communicating with all sorts of apps (not just Airtable). Things that I always found very difficult to do in Zapier are made very simple in Integromat.
For example, whenever you are adding or updating Airtable attachments, you are given the option to specify the filename:
And you can always make your own custom API calls with Integromat as well: