As part of making default apps for lf, I found you can use mimeopen (see man page) to set default apps for mime types. To open a file using mimeopen, the lf shortcut is 'o' ot open, 'O' for ask before open.

Foliate is not available for linuxmint directly. I had to add a repository using command:

sudo add-apt-repository ppa:apandada1/foliate

Foliate is an ebook reader. Better than xviewer (very slow) and fbreader (clunky).