Check ATC().

if atc('xlsx',justext(lcFileName))>0

Just a side note. In your code you compare a comparison against .T., which to someone (me included) looks kind of strange. It's a little bit like *Is it true that it is true". You can change it into.

if 'xlsx' $ Justext(lcFileName)

>is there a function that accepts upper AND lower case - right now my program (excerpt below) only detects lower
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>if 'xlsx' $ Justext(lcFileName)  = .T.
>