Vim NERDTree: show only .txt files?

Using NERDTree plugin, I want to view only *.txt files. There is a NERDTreeIgnore variable, but I want something like NERDTreeWhitelistFilter.

Is there a way to whitelist what I see?

This is what you want:

:let NERDTreeIgnore += ['\(\.txt\)\@<!$[[file]]']