expand "update" for validation #6919

Without this update, the failure case was this:

1. When there are required fields, click Accept without filling them in.
2. Fill in all required fields and click Accept.
3. The form remains and you cannot see the preview.

src/main/webapp/file-download-popup-fragment.xhtml

@@ -175,7 +175,7 @@
                              rendered="#{popupContext == 'previewTab'}"
                              actionListener="#{FilePage.showPreview(guestbookResponse)}"
                              process="previewTab"
-                             update="previewTab">
+                             update="fileForm:tabView">
                 <f:param name="DO_GB_VALIDATION_#{popupContext}" value="true"/>
             </p:commandButton>
             <!--WORLDMAP BUTTON-->