'Assert that the L{TrackingFilePath} C{fp} was used to open one sibling
with the given extension.
@param fp: A L{TrackingFilePath} which should have been used to open
file at a sibling path.
@type fp: L{TrackingFilePath}
@param extension: The extension the sibling path is expected to have
had.
@type extension: L{bytes}
@raise: C{self.failureException} is raised if the extension of the
opened file is incorrect or if not exactly one file was opened
using C{fp}.'

def _assertOneOpened(self, fp, extension):

    opened = fp.openedPaths()
    self.assertEqual(len(opened), 1, 'expected    exactly    one    opened    file')
    self.assertTrue(opened[0].basename().endswith(extension), ('%s    does    not    end    with    %r    extension' % (opened[0].basename(), extension)))
