Test the file_validate_extensions() function.
function testFileValidateExtensions() {
$file = entity_create('file', array(
'filename' => 'asdf.txt',
));
$errors = file_validate_extensions($file, 'asdf txt pork');
$this
->assertEqual(count($errors), 0, t('Valid extension accepted.'), 'File');
$file->filename = 'asdf.txt';
$errors = file_validate_extensions($file, 'exe png');
$this
->assertEqual(count($errors), 1, t('Invalid extension blocked.'), 'File');
}