Setting the fallback extension to "mp4" is indeed not correct.
However setting it to "" might be even better than passing the fallback through the constructors:
 tagLoaderTagLib.Load(strFile, m_tag, ""); will first try to work with the extension from strFile anyway.
So usually the fallback is not used; only if the file has no extension, the fallback is used. But in that case, I'm not sure what the fallback should be; I would think empty.