`list_files` and `list_folders` fails if non-alphanum character contained in substr

**Describe the bug**

In certain cases, users will need to define a `substr` that contains a non-alphanum character. For example, listing all of the `.tiff` files in a folder.

The functions don't handle this functionality correctly, instead returning an empty list.

**Expected behavior**

Add support for this functionality.

**To Reproduce**

Run `list_files` with `substrs='.tiff'`.