docs: add reference to applyAccept from upload() (testing-library#1144)

Author: khitrenovich

docs/user-event/api-utility.mdx

@@ -150,6 +150,9 @@ upload(
 Change a file input as if a user clicked it and selected files in the resulting
 file upload dialog.
 
+> Files that don't match an `accept` property will be automatically discarded,
+> unless [`applyAccept`](options.mdx#applyaccept) is set to `false`.
+
 ```jsx
 test('upload file', async () => {
   render(