Oracle Service Bus - Decoding of MIME attachments from MIME Content-Transfer-Encoding='7bit' not supported

I've developed a proxy service in OSB 12c with MTOM enabled

I can send binary files successfully as pdf, doc, ppt or xls, but as soon as I send a text file the following error arises:

REQUEST:

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:upl="http://UploadFileService.acme/">
   <soap:Header/>
   <soap:Body>
      <upl:scanDocRequest>
         <requestMsg>
            <doc>cid:719461305114</doc>
         </requestMsg>
      </upl:scanDocRequest>
   </soap:Body>
</soap:Envelope>

RESPONSE:

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
   <soap:Body>
      <con:fault xmlns:con="http://www.bea.com/wli/sb/context">
         <con:errorCode>OSB-382118</con:errorCode>
         <con:reason>Decoding of MIME attachments from MIME Content-Transfer-Encoding='7bit' not supported</con:reason>
         <con:location>
            <con:node>AntivirusPipelinePairNode</con:node>
            <con:pipeline>request-a00020f.N696e5fe3.0.15143454755.N8000</con:pipeline>
            <con:stage>ReportingIn</con:stage>
            <con:path>request-pipeline</con:path>
         </con:location>
      </con:fault>
   </soap:Body>
</soap:Envelope>

Response fails with

Decoding of MIME attachments from MIME Content-Transfer-Encoding='7bit' not supported

Content-type = application/octet-stream instead of plain/text solved my problem