itch.io

For the AttachFile method you'll need to provide some kind of file data or a path to the file to send with the report, as outlined in the documentation here. I'm not aware of any non-code ways to provide those parameters. Unfortunately, Unity also does not provide any easy way to let a player browse for files to attach. 

I did find this package that provides a file browser for Windows/Mac/Linux: https://github.com/gkngkc/UnityStandaloneFileBrowser. It also would require some code to use though.

With it, you might be able to create a script like this:

public class AttachFile : MonoBehaviour 
{
     public FeedbackForm form;
     public void GetAttachment()
     {
         var paths = StandaloneFileBrowser.OpenFilePanel("Open File", "", "", false);
         foreach (var path in paths)
         {
             form.CurrentReport.AttachFile(Path.GetFileName(path), path);
         }
     }
}

You could call the GetAttachment method from your button and it should do the rest, but I haven't tested this.