Returns the content type of the specified part.
Applies the content type resolution rules specified in the Open XML packaging specification and returns the content type associated with PartName in the archive.
If there's no content type mapped for the part or for the extension, an empty string is returned.
.NET Standard Notes
To call this method in an asynchronous manner call ResolveContentTypeAsync instead. When the method completes (or an error is encountered) the ResolveContentTypeCompleted event will fire.