$ grep -r -i "string" /your/directory/path/

-r (search recursively)

-i (case insensitive)

-l (list only file names)

--include=\*.${file_extension} – search files that match the extension(s) or file pattern only

grep -r -i -l --include \*.jpg --include \*.png --include \*.gif "Mouse" /your/directory/path/