Directories should have MIME type inode/directory. - #3836

Since os.path.isdir() and inode/directory both call it a directory, I suggest renaming is_folder() to is_directory().

Couple of comments:

1. I am not comfortable with having guess_type perform filesystem I/O in general. Add a parameter named check_for_dirs defaulting to False and when guess_type is called for open actions use it.

2. Add some documentation for this mime/type to docs/open_actions.rst

3. Add a changelog entry for this to docs/changelog.rst

Actually maybe name the parameter allow_filesystem_access rather than check_for_dirs

You can fix the CI failure by having two empty lines above and below the new function.