Use content type application/octet-stream in case it cannot be
determined. See http://docs.oracle.com/javase/7/docs/api/javax/activation/DataSource.html#getContentType():
This method returns the MIME type of the data in the form of a string.
It should always return a valid type. It is suggested that getContentType
return "application/octet-stream" if the DataSource implementation can
not determine the data type.