如何使用 MimeKit/MailKit 设置自定义内容传输编码？

Question

This question is about what I think is "breaking the standard" and I understand that it's possible MimeKit is setup to specifically not allow me to do the things I'm asking. These custom messages will be only used internally and not for regular email sending.

The below is the attachment I'm able to create using the basic functions:

Content-Type: application/octet-stream; name=example.txt
Content-Disposition: attachment; filename=example.txt
Content-Transfer-Encoding: base64

**BASE64 ENCODED ATTACHMENT**

What I would like to know is if it's possible to create the following:

Content-Type: application/octet-stream; name=example.txt; type=****
Content-Disposition: attachment; filename=example.txt
Content-Transfer-Encoding: *****

**CUSTOM ENCODED ATTACHMENT**

Where I have a custom string to set the "Content-Transfer-Encoding", possibly a custom "type" under "Content-Type", and also use my own custom code to encode the message.

I'm assuming that the easiest way to custom encode my message is to do this outside of MimeKit, and then set MimeKit to not encode. Is there anyway that I can create custom headers that just contain strings I want?

Extra Question:

How would I go about changing:

Content-Type: application/octet-stream; name=example.txt
Content-Disposition: attachment; filename=example.txt

to:

Content-Type: application/octet-stream; name="example.txt"
Content-Disposition: attachment; filename="example.txt"

Answer 1

As you've discovered in your own answer, you can override the Content-Transfer-Encoding header by using Headers.Replace() or even Headers.Add() if you don't set the ContentTransferEncoding property.

I guess your other main question is how do you get the custom encoded content?

Instead of doing:

Content = new MimeContent(File.OpenRead(file), ContentEncoding.Default),

All you need to do is pass in a pre-encoded stream into the MimeContent .ctor (and continue using ContentEncoding.Default as the second parameter).

If you need to set a type parameter in the Content-Type header, you can do this:

attachment.ContentType.Parameters.Add("type", "value");

or

var parameter = new Parameter ("name", "value");
attachment.ContentType.Parameters.Add (parameter);

or

attachment.ContentType.Parameters["type"] = "value";

The only question remaining is how to enforce parameter values being quoted. For that, you are out of luck. MimeKit will only quote the value if it needs to be quoted due to the characters in the value.

Answer 2

I found my one of my answers is replacing headers:

var attachment = new MimePart("application", "octet-stream")
{
    Content = new MimeContent(File.OpenRead(file), ContentEncoding.Default),
    ContentDisposition = new ContentDisposition(ContentDisposition.Attachment),
    ContentTransferEncoding = ContentEncoding.Base64,
    FileName = Path.GetFileName(file),
};

attachment.Headers.Replace("Content-Transfer-Encoding", "******");