After a few searches from Google, what I come up with is:
find my_folder -type f -exec grep -l \"needle text\" {} \\; -exec file {} \\; | grep text >
find my_folder -type f -exec grep -l \"needle text\" {} \\; -exec file {} \\; | grep text
find . -type f | xargs file | grep "ASCII text" | awk -F: '{print $1}'
Use find command to list all files, use file command to verify they are text (not tar,key), finally use awk command to filter and print the result.