If you are familiar with mhn directives, you can enter them into your message. Select the Attempt MHN checkbox item under the More... menu. You may want to disable Format mail to make sure that long mhn directives are not chopped into multiple lines at the wrong places.
To create a multipart MIME message interactively, use the Insert Part... menu item. This adds another part to your message and promotes it to MIME content-type multipart/mixed if necessary. The first time you add a part, you have the option of preserving the existing message body or completely replacing it. When the file is inserted you are asked to choose its MIME Content-type: and Content-transfer-encoding:.
The Insert External... menu entry is used to create a MIME part of content-type message/external-body. This is an indirect reference to a file -- either a local file or a file available via anonymous FTP. A dialog lets you fill in the various parameters to the indirect reference. For an anonymous FTP pointer, the Site is the Internet host name, and the Directory and File name determine the file. The Transfer mode is either "text" or "image." Use "image" to ensure that all 8 bits of each byte get transferred. The Description is for the person who receives the mail message; it should describe what the external part contains.
Tip: A file section dialog appears first, but this is not always appropriate if you are making an external reference to a non-local FTP site. Just hit Cancel on the file selection dialog. Then, when you enter the file name, the content-type will be inferred from the name. You can always override the content-type if exmh guesses wrong.