It looks like there isn't any explicit support in Pipelines for deleting a file attachment.
You could use the "Make Request" action in the Quickbase Channel under "Quickbase APIs" for this. This basically allows you to create your own custom Quickbase Pipeline Action (You just have to know enough about the API to form the call)
@Mark Shnier (YQC) has done some of the heavy lifting for you in his reply, if you wanted to use the HTTP API instead of the Restful API.
@Mark Shnier (YQC)
Do you know if Quickbase going to be deprecating Webhooks and Actions along with Automations? Or will those be here to stay?