Multiple form-data file upload parameters

You are on track. I have a slightly different approach you can try out.On the client, try something like:

this.uploader = new FileUploader({
            url: url,//The enpoint you are consuming
            additionalParameter: {
                issueId: result.data.id //your parameter-remove quotes
            },
            headers: [{ name: 'Accept', value: 'application/json' }],//your custom header
            //autoUpload: true, //configure autoUpload
        });

The library also has onErrorItem and onSuccessItem callbacks that you can leverage like below:

this.uploader.onErrorItem = (item, response, status, headers) => this.onErrorItem(item, response, status, headers);
this.uploader.onSuccessItem = (item, response, status, headers) => this.onSuccessItem(item, response, status, headers);

Then(Optional) - Callbacks:

    onSuccessItem(item: FileItem, response: string, status: number, headers:ParsedResponseHeaders): any {
            //this gets triggered only once when first file is uploaded       
     }
    onErrorItem(item: FileItem, response: string, status: number, headers: 
           ParsedResponseHeaders): any {
                let error = JSON.parse(response); //error server response            
            }

On the API side you can restructure like below - Change it to your own signature.

 public async Task<IActionResult> UploadImages(MyFile upload)

Then the MyFile model can be something like:

public class MyFile
    {
        public string issueId { get; set; }        
        public IFormFile File { get; set; }
    }

To get the param and the file:

var file = upload.File //This is the IFormFile file
var param = upload.issueId //param

To save the file to disk:

using (var stream = new FileStream(path, FileMode.Create))
        {
            await file.File.CopyToAsync(stream);
        }