GeezerVison

Extensions that don't start with a dot will never be found by
methods/functions that act on the registry, so we should stop users from
mistakenly adding them.

The one exception is the empty string extension.  This could be in use
to detect absent extensions, so instead of raising a ValueError we emit
a warning.

…mimetypes

encukou left a comment

Co-authored-by: Petr Viktorin <[email protected]>