custom value for boundary in Content-Type header

Hmm, if this was a SOAP request then there's an option for "Inline Files" but there isn't the same option for REST.

In the source code (com.eviware.soapui.impl.wsdl.submit.transports.http.support.attachments.AttachmentUtils), the implementation for SOAP boils down to this, which is fairly straightforward:

  //...
        String content = null;
        byte[] data = Tools.readAll(in, -1).toByteArray();

  //...
            content = new String(data);
  //...

So, you can use the workaround Paul referred to, plus use an inline Groovy script to inline the file contents:

Content-Type: multipart/form-data; boundary="----someboundary"

------someboundary
Content-Type: application/octet-stream; name="audio.mp3"
Content-Transfer-Encoding: binary
Content-Disposition: form-data; name="audio.mp3"; filename="audio.mp3"

${=new String(new File('audio.mp3').bytes)}

Note that the boundary value in the body needs two more hyphens than the boundary value in the header.

To my eye, the request generated seems identical to a normal REST request with attachments, except for the boundaries. Plus if I send it to a local MockService it seems to parse it ok. So there's every chance this might work for you.