p：fileUpload required =＆＃39; true＆＃39;不工作

Question

<p:fileUpload fileUploadListener="#{expeditionGroupView.handleFileUpload}" mode="advanced" dragDropSupport="true"
                      update="upload-messages" sizeLimit="10000000" allowTypes="/(\.|\/)(gif|jpe?g|png)$/" 
                      cancelLabel="#{lbl['label.cancel']}" uploadLabel="#{lbl['label.send']}" label="#{lbl['label.choose.file']}"
                      invalidFileMessage="#{msg['error.invalid.file.type']}" required="true"/>

我正在尝试使用required =＆＃34; true＆＃34;验证我的p：FileUpload，为什么它不起作用？