Notebook files extension check is case sensitive

In case a notebook file extension is not all lower case (e.g. NOTE.IPYNB) then notebook will identify it as plain text and not as a notebook.

I fixed this issue in the PR [https://github.com/jupyter/notebook/pull/4355](https://github.com/jupyter/notebook/pull/4355)

Review the linked pull request #4355 first, then trace the notebook filename extension check it changes. Confirm that extensions such as NOTE.IPYNB are recognized as notebooks rather than plain text, and run the relevant tests from that pull request to verify the behavior.

Written by the indexing model from the issue text.