To find which pure text files contains a given text from the command line, this is the proper command:
grep -ilr "pattern" directory
-i option makes the search case-insensitive,
-r searches the pattern recursively (that is, in the subdirectories that the actual directory might contain), and
-l shows the file list where the pattern occurs.
Thanks to Juan Luis Belmonte for the tip (credit where credit is due).