REST service multipart/mixed request problem

Hi!

I want to send the below multipart/mixed type of request from SoapUI but I am unable to add the Content-MD5 header to the second part of body. This header is needed, otherwise the request will fail.

My question is: How to add custom headers into a part of multipart request body?

Raw request taken from Fiddler, when response is OK:

POST http://srv2012r2:8080/simo/api/documents HTTP/1.1
...

Content-Type: multipart/mixed; boundary=19D523FB
Host: srv2012r2:8080
Content-Length: 11979
Expect: 100-continue

--19D523FB
Content-Type: application/json

{

  "fields":[{"Key":"e9e269b9-867f-4cf6-946f-db49091cd52d","Value":"Simplest document ever.docx"}],

  "storageappid":"d9ca0630-e4f8-4112-a411-ff04d89f2225"

}
--19D523FB
Content-Type: application/octet-stream
Content-Disposition: attachment; filename="Simplest+document+ever.docx"
Content-MD5: EFV8OUymEQRJLoppzOjQ1w==

PK   ! ߤÒlZ
  [Content_Types].xml ¢(   ´”ËnÂ0E÷•ú‘·Ub袪*‹>–-Ré{Vý’Ǽþ¾QU
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...

--19D523FB--