In Moodle, since 2014, it is not possible to add custom file types when one is defined in
config.php, which is the case in MoodleBox for about 2 years (added in v3.9.0, published nov. 8, 2020).
doxendine I added a couple custom file types in the past.
I suppose you did it with an older version?
doxendine Do you know if this is added somewhere else now?
You should first comment out in
config.php the whole definition of
$CFG->customfiletypes, then add your custom types, then re-enable the variable definition. If you don't need your users to download the certificate, you can also leave it commented or remove it totally.
PS. This behaviour is a bit annoying, I'll try to find a long term solution.