How to add another filter to "set mask"

[Larry Alkoff <labradley@xxxxxxxxxxxxxx>]

I would like to add another filter to the default
# default: set mask="!^\\.[^.]"
in ~/muttrc that stops display of files starting with '.'.

I wish to _not_ display any file ending in .msf.  These are Thunderbird 
index files that should not be displayed when using Mutt.

So the filter would be something like:
"!default_mask or !.msf$"
but I can't find the documentation on the correct syntax.

Would someone please help me out on the syntax?

Larry

--
Larry Alkoff N2LA - Austin TX
Using Thunderbird on Slackware Linux