minor #10803 [TESTING] File upload - Multiple files case (Guikingone)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes #10803).

Discussion
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[TESTING] File upload - Multiple files case

Hi everyone,

I've noticed that the documentation lack this type of example which can occur many times in a project (especially if you have some CRUD).

Thanks for the reviews 😃

Commits
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c563c37 fix(FileUpload): Addition on multiple files

Author: javiereguiluz

testing.rst

@@ -749,6 +749,10 @@ their type::
     // uploads a file
     $form['photo']->upload('/path/to/lucas.jpg');
 
+    // In the case of a multiple file upload
+    $form['my_form[field][O]']->upload('/path/to/lucas.jpg');
+    $form['my_form[field][1]']->upload('/path/to/lucas.jpg');
+
 .. tip::
 
     If you purposefully want to select "invalid" select/radio values, see