Typo in mime type variable

Found in app/resources/mime_types.py

    XLXS = MimeType(
        type="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
        extension=".xlsx",
        alternative_types=[],

variable name should be ...

    XLSX = MimeType(
        type="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
        extension=".xlsx",
        alternative_types=[],