p:fileUpload doesn't work inside p:dialog

I'm trying to upload a file using PrimeFaces p:fileUpload inside p:dialog but it doesn't work

<h:form id="form3"> 
       <p:commandLink value="upload" oncomplete="PF('Dialog').show()" />            
       <p:dialog  widgetVar="submitDialog" modal="true"  >
              <h:form id="form" enctype="multipart/form-data" > 
                     <h:panelGrid id="submitPanelGrid" columns="2"  >
                         <p:fileUpload  id="upload" value="#{bean.file}"  mode="simple" sizeLimit="100000"  />
                         <p:commandButton id="btn3" action="#{bean.submit()}"  icon="ui-icon-circle-check" ajax="false"  /> 
                     </h:panelGrid>
                 </p:panel>
              </h:form>
       </p:dialog>
</h:form>   

I'm getting this exception just after I click on the link:

org.apache.tomcat.util.http.fileupload.FileUploadBase$InvalidContentTypeException: the request doesn't contain a multipart/form-data or multipart/mixed stream,
content type header is application/x-www-form-urlencoded;

But outside <p:dialog>, it works just fine.