This is an update about the External Request in the Action Step.

You can include a file itself in the API calls instead of the URL link of the file.

In the External Request, go Body, multipart/form-data and you will see an Upload Files section. Put a file direct link here to include the file itself in the parameter. You can put the link you get from a Question Step here. (Please note that the file URL link from a Question Step is only valid for 24 hours)