SOAP request multiple attachments

I have java app with SOAP services, so i am trying to attach multiple xml files in request and after that to use them.

I searched a lot and i still can not find the answer.

This is my request:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" 
xmlns:ser="http://services.ls.com.test.si/">
 <soapenv:Header/>
  <soapenv:Body>
    <ser:import>
     <!--Optional:-->
     <RequestMessage>
        <Header>

        </Header>
        <!--Zero or more repetitions:-->
        <files>
           <!--Optional:-->
           <file>cid:file1.xml</file>
           <file>cid:file2.xml</file>
        </files>
     </RequestMessage>
  </ser:import>

 

The error i am facing is:

faultstring>No attachment for  id file1.xml found in [file2.xml]</faultstring>