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`PR 225 Genelist upload (C4-875) <https://github.com/4dn-dcic/snovault/pull/225>`_
Instrumentation added to help debug C4-875.
* Improved error messages for ``ValidationFailure`` in ``attachment.py``.
Actual proposed fix:
* In ``attachment.py``, replaced ``mimetypes.guess_type`` with new function ``guess_mime_type``
(adjusting the receipt of return value, since I adjusted that slightly to return the mime type,
not a tuple of mime type and encoding).
* Make sure that we have useful return values for common file extensions.
Opportunistic:
* Better ``.flake8`` file excluding a bunch of whitespace-related issues we don't need to care about yet.
* Add a lint target to the ``Makefile``.
* Suppress an annoying warning from the ``jose`` package (included by ``moto 1.3.7``)
about how it's not going to work in Python 3.9.
* Do keyword-calling of ``ValidationFailure`` in ``attachment.py`` just to clarify what the weird args are.
* Add an extra warning message in ``create_mapping.py`` for certain unusual argument combinations.
(This had come up elsewhere in a discussion I had with Will and was just waiting for a PR to ride in on.)