BizTalk incorrect request (multipart form data) - multipartform-data

Question

We need to send a file of xbrl format (a type of xml) to a receiving system. However, the receiving system requires that we are sending some form data as well. Regarding the xbrl file we need to send with it a form data, name="data".
Regarding the second part, we need to send an email to the system via form data. And that is name="email".
(I'll be the first to admit that I really do not understand form data at all so I cant explain much better than this).
I have a custom pipeline and in the encode stage I have put my pipeline component (the entire Execute method is down below) and after that a MIME/SMIME component (standard Microsoft).
Take a look at how the request looks like when it is being sent from Postman (and fully accepted by the receiving system):
Content-Type: multipart/form-data; boundary=--------------------------035085236562027409735938
Content-Length: 299420
----------------------------035085236562027409735938
Content-Disposition: form-data; name="data"; filename="Dk_{5AA9547A-E103-40CD-A2F4-6B77F010E570}.xbrl"
Content-Type: application/xml
<?xml version="1.0" encoding="utf-8"?>
<!- ---- Lots of removed xbrl ---- ->
</xbrli:xbrl>
----------------------------035085236562027409735938
Content-Disposition: form-data; name="email"
Content-Type: text/plain
test#test.com
----------------------------035085236562027409735938--
Now what you see below is what we have managed to achieve this far:
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="_E9E8D52C-4722-488D-9FEC-713446D4A5C4_"
--_E9E8D52C-4722-488D-9FEC-713446D4A5C4_
MIME-Version: 1.0
Content-Type: application/xml; charset="UTF-8"
Content-Transfer-Encoding: 7bit
Content-ID: {E8CA3F1A-F953-4FFB-A054-DEB8466A5D86}
Content-Description: body
<?xml version="1.0" encoding="utf-8"?>
<!- ---- Lots of removed xbrl ---- ->
</xbrli:xbrl>
--_E9E8D52C-4722-488D-9FEC-713446D4A5C4_
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 7bit
Content-ID: {3B35ECAC-990D-458C-8E7E-9C178CA99988}
Content-Description: email
Content-Disposition: attachment; filename="=?utf-8?B?QXR0YWPobWVudDF=?="
test#test.com
--_E9E8D52C-4722-488D-9FEC-713446D4A5C4_--
Now, a quick comparison reveals that we are closing in on the solution. The first important issue must be that the Content-Type (first part below MIME-version: 1.0) is becoming multipart/mixed when we need multipart/form-data.
Second issue is in the Content-Disposition which for both parts does not, at all, look like expected.
Please take a look at my pipeline component. What you see is the entire code from the Execute. Since formDataPart.Data only takes a stream I have a method that translates a string to a stream (MemoryStream).
// Variables
var outMsg = pContext.GetMessageFactory().CreateMessage();
outMsg.Context = PipelineUtil.CloneMessageContext(pInMsg.Context);
outMsg.Context.Promote("ContentType", "http://schemas.microsoft.com/BizTalk/2003/http-properties", "multipart/form-data;");
// Process xbrl file
var msgPart = pContext.GetMessageFactory().CreateMessagePart();
msgPart.PartProperties.Write("FileName", "http://schemas.microsoft.com/BizTalk/2003/mime-properties", "SomeID.xbrl");
msgPart.PartProperties.Write("ContentDescription", "http://schemas.microsoft.com/BizTalk/2003/mime-properties", "xml");
//msgPart.PartProperties.Write("ContentTransferEncoding", "http://schemas.microsoft.com/BizTalk/2003/mime-properties", "7bit");
msgPart.Data = pInMsg.BodyPart.Data;
msgPart.ContentType = "application/xml";
msgPart.Charset = "UTF-8";
outMsg.AddPart("data", msgPart, true);
// Form data (email)
var formDataPart = pContext.GetMessageFactory().CreateMessagePart();
formDataPart.ContentType = "text/plain";
formDataPart.Charset = "utf-8";
formDataPart.PartProperties.Write("ContentType", "http://schemas.microsoft.com/BizTalk/2003/http-properties", "form-data;");
formDataPart.Data = GenerateStreamFromString("test#test.com");
outMsg.AddPart("email", formDataPart, false);
return outMsg;
Does anybody know whow we can achieve this? I can also mention that we are using the MIME/SMIME component after this custom pipeline component.
I really think that the solution from Greg Forsythe would be the best solution since I would have total control over the message but I have no idea how to translate that into actual code.
Greg_suggestion
UPDATE: After so many trials and not coming anywhere we decided to ditch the above solution and go with Gregs solution instead. There is an issue there so I hope some C# guru can help us out. Please take a look at the new question if you are struggling with the form data request issue.
Second attempt here