List file names to text files according their extensions

Example

kumbasar@ghost ~/g/filelistener> ls
LICENSE         README.md       filelistener.py volkan.mp3      volkan.mp4      volkan.pdf      volkan2.pdf     volkan3.pdf
kumbasar@ghost ~/g/filelistener> ./filelistener.py
kumbasar@ghost ~/g/filelistener> cat pdf_file.txt
volkan.pdf
volkan2.pdf
volkan3.pdf
kumbasar@ghost ~/g/filelistener>