FileType magic sequence with offset cannot be recognised

[andrea-quartini]

If a file type magic sequence have an offset, it cannot be recognised because the offset is applied to the magic byte sequence as well; see for instance Mp4 and M4v file types

Steps to reproduce the behavior:

1. Into the FileTypeValidatorTests.cs, add a test case for Mp4 file type and file name "test.mp4"
2. Run the test
3. Test throws an exception on GetFileType method