JSF dialog with fileupload close when commandbutton (ajax= false) enter

Question

I'm using jsf 6.1.

I have a <p:dialog and a <p:fileUpload inside. When the <p:commandButton with ajax= false is fired, the dialog will be closed. I want if any validation errors happened, the diaglog will not be closed. Can anyone help me: 1.Explain why dialog close when use ajax=false 2.The way to prevent closing dialog when validation error.

My sample code:

<h:form id="form" enctype="multipart/form-data">
    <p:inputText value="#{bean.value}" />
    <p:fileUpload value="#{bean.reportFile}" mode="simple" skinSimple="true"/>
    <p:commandButton action="#{bean.updateExp()}" ajax="false"/>
</h:form>