yes, if you use the default extension but who's to say that it will be the case on every machine, for every customer....
I have often seen at customer sites that they use an extension for the masters and another for the xRefs so they can visually see in windows Explorer what kind of file it is (I'm not saying it's the right way to go but it's one way.... )
As a rule of the thumb, I never depend on the extension of a file to determine what it is. For example, .DOC is used by a big lot of software and it would be a big mistake to assume it's Microsoft Word.