As of the time of this writing, Using the automation for Jira webhooks does not support files to be uploaded to the Jira issue automatically.
There is also no way to get Jira to download the file onto the ticket as part of the automation.

As such, I suggest that the automation for Jira system provide an action for downloading a file and attaching it to the Jira ticket and/or an action for Base-64 decoding a string as a file and attaching it to the Jira ticket.