Why would you want to set the mime type? Are you saying there is a bug and mime types are not detected correctly?

I want to set the MIME type because Apple uses at least 1 unregistered MIME type (application/x-apple-aspen-config) and I want to email one of this things. If you want to learn more, search on "mobileconfig mime type"

No, I am not saying anything about bugs, I am just asking if there is a way to specify the MIME type on an attachment. Is there?

FYI, the Linux "mailx" program has this capability: edit /etc/mime.types or ~/.mime.types

Another way to think about this is how do you attach a file that does not have the "correct" file suffix?

thanks, steve

while a direct edit of mimetypes.rdf might get you there, I think perhaps it might be better if you arranged to "receive" such an email from an apple device as it will already be encoded with a mime type and open the attachment in Thunderbird, making the defaults as you go. My little understanding is Thunderbird will use the information in mimetypes RDF created from the opening action to encode future emails. But I am on thin ice.

My reading up on this talks about placing these configuration files on servers, so where does email come into it?

The actual config file looks like a plain text xml file, so doesn't need any exotic encoding if you just want to convey it to someone else. I haven't yet encountered a use case for deploying via email.

I agree that if I choose to pursue my question "mimetypes.rdf " is worth investigation (though it is probably just for the receiving end of things. Let's consider my question as closed with the answer being "probably not".