Only testing .txt files does not cover enough test cases

Line 12 in b1345a5

@pytest.mark.parametrize('input_file', ['tests/data/structured/Documentation '

Going through old issues due to new activity on Github.

We now have

Line 220 in f730f40

def test_import_object_cases(testpath, input_file, expected_mimetype,

which tests more file types. Even more tests are at https://github.com/Digital-Preservation-Finland/file-scraper which is now used by import-object.