listfileext is a small Python script that lists all file extensions in a given directory and all it's sub-directories (recursivly).
If you want to know e.g. what for (obscure) files are in your documents folder, open a terminal and use this script like this:
listfileext.py ~/Documents
As you see, you can use any path as parameter.
Hidden files
By default hidden directories and files (like .git or .gitignore) are skipped, if you want to scan for those also, just open the script in a text editor of your choice and set the value of skip_hidden_items from True to False.