Determining what mailcap entry mutt is trying to run for an attachment?

[Jon Leech]

    I just upgraded from mutt as packaged for Debian 9 (1.7.2, I think)
to mutt 1.10.1 packaged for Debian 10. Now attempting to 'v'iew a
text/html mail attachment just displays it as text on my console. I've
made no change to my ~/.mailcap, which is where the matching action
should be found, and the underlying chromium executable is definitely in
my search path:

text/html                ; chromium %s & sleep 2     ; nametemplate=%s.html

    Is there a way to get mutt to log what action it tries to take in
this situation, or otherwise diagnose what (isn't) happening?

    Jon