The onMessage element doesn't contain variable type attribute.
The type is defined by the message type of the selected operation. 

See schema:
<pick createInstance="yes|no"? standard-attributes>
     standard-elements
     <onMessage partnerLink="ncname" portType="qname"?
          operation="ncname" variable="ncname"? 
          messageExchange="ncname"? >+
         <correlations>?
             <correlation set="ncname" initiate="yes|join|no"?/>+
        </correlations>
        <fromPart part="ncname" toVariable="ncname"/>*
        activity
    </onMessage>
    <onAlarm>*
          ( <for expressionLanguage="anyURI"?>duration-expr</for> |
            <until expressionLanguage="anyURI"?>deadline-expr</until> )
         activity
    </onAlarm>
</pick>

I believe this issue was filed for onEvent element.
OnEvent element is mentioned in Summary.

Description has wrong mentioned of OnMessage element.

I'm sorry ... It was my mistake in description. The bug filed for onEvent 
element.
      
<onEvent partnerLink="NCName" portType="QName"?
      operation="NCName" (messageType="QName" | element="QName")?
      variable="NCName"? messageExchange="NCName"? >*
   <correlations>?
      <correlation set="NCName" initiate="yes|join|no"?/>+
   </correlations>
   <fromPart part="NCName" toVariable="NCName"/>*
   <scope ...> ... </scope>
</onEvent>

Value for messageType attribute is provided automatically when variable 
attribute is set. It calculated from the value of "operation" attribute.

But this logic should be moved to BpelOM after beta. Current implementation 
will not update the messageType attribute if operation was changed via source 
editing or by message flow link drawing tool.

ok ... attribute provided automatically