Email is the emperor of asynchronous text-based communication. The formats are well defined and tiny, and adoption is very high.
Mailcap specifies which extensions to use to open files with. Out of the box
mutt performs horribly with anything but plain-text. Here's an example
text/plain; cat %s; copiousoutput text/html; links -dump %s; nametemplate=%s.html; copiousoutput application/pdf; /usr/bin/evince %s text/pdf; /usr/bin/evince %s image/*; feh %s
Probably the best way of securing email is to host your own provider. After the lavabit shutdown, it's probably a good option to use riseup.