Content-Transfer-Encoding changed to Base64 after being signed

[Jimex]

Describe the bug
I have a content part with Binary Content-Transfer-Encoding, but it switches to Base64 after being signed. Did I miss anything?

Platform (please complete the following information):

• OS: [e.g. Windows
• .NET Runtime: CoreCLR
• .NET Framework: .Net 8
• MimeKit Version: 4.3.0

To Reproduce
Steps to reproduce the behavior:

1. Create a MimePart with Binary Content-Transfer-Encoding
2. Used MultipartSigned.Create to sign the part

Expected behavior
The contentPart's Content-Transfer-Encoding should remain the same after signed.

Code Snippets

var signPart = MultipartSigned.Create(SecureMimeContext, signer, contentPart);

Update****
It worked after I change to the codes below.

 var signPart = new MultipartSigned();
 signPart.Add(contentPart);

 ApplicationPkcs7Signature signature;
 using (var memory = new MemoryBlockStream())
 {
     // var prepared = Prepare(entity, memory);
     contentPart.WriteTo(memory);
     memory.Position = 0;

     // sign the cleartext content
     signature = ctx.Sign(cmsSigner, memory);
 }

 // add the detached signature as the second part
 signPart.Add(signature);

[jstedfast]

It changes the Content-Transfer-Encoding to base64 because the binary transfer encoding often gets forcefully changed to base64 in transit if any SMTP server along the way does not support the BINARYMIME extension.

I don't consider this a bug.