feat(io_interfaces): add file extension to index_list

[SherlockMones]

Add the image_format as file extension to the index_list if it's missing. That way, users can use the index list without manually appending the file extension.

[muellerdo]

Is this not automatically done if you add the image_format as a parameter to your DataGenerator?

[SherlockMones]

Sorry about that, did not yet know about the image_format param in DataGenerator. Reverted the changes and improved documentation (added warning) and example (formatting and including image_format param).