Mutating MIME parts with Thunderbird extension

Question

I have a JavaScript function which operates on strings, and I’d like to write a Thunderbird extension to apply it to every MIME part in an outgoing message – both inline parts like the body and any attachments. However, I can’t find the API call I need to request the MIME data from Thunderbird. Ideally, I’d write something like

var parts = GetMimeParts();
for (var i = 0; i < parts.length; i++) {
    parts[i].data = mutate_string(parts[i].data);
}

What Thunderbird API should I use to retrieve the MIME parts from an outgoing message?

Answer 1

The tl;dr is, there's no simple API to do that. You would have to:

• obtain the MIME structure of the original message (easy, using MsgHdrToMimeMessage, see http://blog.xulforum.org/index.php?post/2011/03/14/Basic-MimeMessage-demo)
• reserialize it (feasible, I know the tests in Thunderbird do just that, you would probably have to convert a MimeMessage into a SyntheticMessage, and use toMessageString (http://mxr.mozilla.org/comm-central/source/mailnews/test/resources/messageGenerator.js#664), or maybe just write the serialization yourself ; in any case, you'd probably lose a lot of information (whitespace, etc.), as well as possibly attachments (MsgHdrToMimeMessage will drop them if they're too big)),
• replace the old message with the new one (known to be feasible, see my answer on how to do nsIMsgDBHdr permanent change commits)

What exactly are you trying to achieve?