Ensure form is submitted as multipart/form-data in case a file field might be added dynamically

Author: Dominic Tubach

src/Form/Control/ArrayArrayFactory.php

@@ -85,6 +85,16 @@ public function createFormArray(
     );
 
     if (0 === $numItems) {
+      if (0 !== $definition->getMaxItems()) {
+        // There might be a file field in the child elements. To ensure that the
+        // form is sent as multipart/form-data in case a file field is
+        // dynamically added, we claim that the form has a file element.
+        // If there's no file field initially, the form would be sent as
+        // application/x-www-form-urlencoded, i.e. file contents wouldn't be
+        // submitted.
+        $formState->setHasFileElement(TRUE);
+      }
+
       // Ensure we get an empty array if there's no item.
       $form[] = [
         '#type' => 'hidden',