Explain why we're using XHR here (#30187)

files/en-us/web/api/file_api/using_files_from_web_applications/index.md

@@ -380,7 +380,11 @@ Here is a live demo of the code above:
 
 ## Example: Uploading a user-selected file
 
-Another thing you might want to do is let the user upload the selected file or files (such as the images selected using the previous example) to a server. This can be done asynchronously very easily.
+This example shows how to let the user upload files (such as the images selected using the previous example) to a server.
+
+> **Note:** It's usually preferable to make HTTP requests using the [Fetch API](/en-US/docs/Web/API/Fetch_API) instead of {{domxref("XMLHttpRequest")}}. However, in this case we want to show the user the upload progress, and this feature is still not supported by the Fetch API, so the example uses `XMLHttpRequest`.
+>
+> Work to track standardization of progress notifications using the Fetch API is at <https://github.com/whatwg/fetch/issues/607>.
 
 ### Creating the upload tasks