Syntax Issue with Create Unassociated Attachment Endpoint

I am trying to use the Create Unassociated Attachment endpoint to upload attachments to my Help Center. This works through Postman if I use the file property and link to a file on my local computer.

The issue is I am trying to do this with files from the web. When I set that same file property to be the publicly-accessible URL of an image, I get the following error. This is the entire response in Postman:

{
    "errors": {
        "file": [
            "cannot be blank"
        ],
        "base": [
            "A file must be provided"
        ]
    }
}

I'm sure the issue is syntax-related, but I'm having trouble finding documentation with correct examples. The example in the developer documentation doesn't seem to reference a file from a URL.

The only other community post I've found on the topic is from 2015, and it doesn't seem like the issue was resolved there.