When a .ttf font is attached to a message, the multipart header does not contain a Content-Type and so receiving applications do not recognise the attachment.
Content-Disposition: attachment; filename=Test-Regular.ttf
This seems to be because the top level font mimetype is not recognised by Claws-Mail and the data is treated as unknown/ttf and that results in no Content-Type being output.
Here (latest git in Debian stable) ttf files are attached with MIME type “application/x-font-ttf”.
Are you sure you have an up to date MIME database in your system?
In Ubuntu 18.04 /usr/share/mime/globs has font/ttf:*.ttf
I remember there being something about giving .ttf a proper mimetype rather than a private use code and this must be it.
BTW here's the list of what's in that new font/ top level:
I wonder if we just say: unknown maps to much like application but with the mimetype just copied in?
this is not a claws-mail bug.