Data file content for Postman Runner to test a POST request of type multipart/form-data

Question

What is the data file that should be provided in Postman runner to execute multiple calls to a POST endpoint that expects multiplart/form-data?

I have an Azure function which looks like this (simplified for this post):

[FunctionName("UploadImage")]
    public static async Task<IActionResult> Run(
        [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = null)] HttpRequest req, ILogger log)
    {
        var form = await req.ReadFormAsync();
        if (!form.Files.Any())  return new BadRequestObjectResult("form.Files is empty");
        var uploadedFile = form.Files.First();
        form.TryGetValue("fileName", out var fileNames);
        return new OkObjectResult($"Uploaded image length: {uploadedFile.Length} file name {fileNames.First()}");
    }

I use Postman to send it a request successfully like this: Postman request body params

I want to call this function 100 times concurrently to test its performance using the "Run Collection" selection of Postman. It asks to provide a csv datafile. So I tried a file that has only 2 lines like this:

FileName, Photo
Attachment1, Regina.jpg

But it does not work with that data file. I get a response message that "form.Files is empty"

How to correctly provide a data file for this Postman runner to test calling the endpoint (n) times async?