diff --git a/src/BeSimple/SoapCommon/Converter/SwaTypeConverter.php b/src/BeSimple/SoapCommon/Converter/SwaTypeConverter.php
index 40d6372..138b973 100644
--- a/src/BeSimple/SoapCommon/Converter/SwaTypeConverter.php
+++ b/src/BeSimple/SoapCommon/Converter/SwaTypeConverter.php
@@ -83,7 +83,7 @@ class SwaTypeConverter implements TypeConverterInterface, SoapKernelAwareInterfa
 
         $this->soapKernel->addAttachment($part);
 
-        return sprintf('<%s href="%s"/>', $this->getTypeName(), $contentId);
+        return sprintf('<%s href="%s"/>', $this->getTypeName(), 'cid:' . $contentId);
     }
 
     /**