problem regarding upload button

Hi,

 if i have not entered any thing in upload button it has to throw exception.
 if you upload file only ,the error has to be dis appeared.how we can do
 in xforms.

if i write codelike this ,even i uploaded also error is not disappeared.what is the problem plz help me.

 <xforms:bind nodeset="//file" constraint=". != ''"/> 

 

 

 <xforms:upload ref="instance('main-instance')/file">
                <xforms:filename ref="@filename"></xforms:filename>
               <xforms:mediatype ref="@mediatype"/>
        <xxforms:size ref="@size"/>  
           </xforms:upload>

 

thanks and regards,

laxminarayana.