WW-5371 Document how to use the new file upload logic

core/src/main/java/org/apache/struts2/action/UploadedFilesAware.java

@@ -33,7 +33,7 @@ public interface UploadedFilesAware {
      * Notifies action about the multiple uploaded files, when a single file is uploaded
      * the list will have just one element
      *
-     * @param uploadedFiles a list of {@link UploadedFile}.
+     * @param uploadedFiles a list of {@link UploadedFile}, cannot be null. It can be empty.
      */
     void withUploadedFiles(List<UploadedFile> uploadedFiles);
 

core/src/main/java/org/apache/struts2/dispatcher/multipart/JakartaStreamMultiPartRequest.java

@@ -48,6 +48,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.UUID;
+import java.util.stream.Collectors;
 
 /**
  * Multi-part form data request adapter for Jakarta Commons FileUpload package that