If you want to upload a file (to Bubble storage), you need to encode it in base64 and set the json as:

{
 "filename": "String"
 "contents": "Base64-encoded binary data"
 "private": Boolean
 "attach_to": "String"
}

An option if you cannot do that, is to use Backend WF instead. Send the URL and in the field, use url’saved to Bubble storage (I think they renamed it two weeks ago, before it was Saved to s3)