I'm using mutt as mail client.  As sometimes HTML attachments are only usable
via a real browser :(   I added an entry in my .mailcap, following the mutt
documentation:

text/html; vivaldi '%s'; nametemplate=%s.html; test=test -n "$DISPLAY"

(the reason for the nametemplate is that vivaldi only renders files if they end
in .html)

With this, mutt saves the file as /tmp/mutt-<host>-<UID>-<PID>-<random> and
then calls "vivaldi /tmp/mutt.html" which of course results in a 'file not
found' :((

I have also tried nametemplate='%s'.html
That expands to /tmp/_mutt_.html

Without the nametemplate definition it properly starts vivaldi with the correct
file name (but it isn't rendered then....)

mutt-1.14.4 on TW 20200804