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Spiders work like a spider web crawling the different directions.
A robot comes into a site on a referral whether that is from another site or a submission.
In that regard the spider can NOT have an extension preference.
The same applies as it travels your site of the links it picks up from the page it comes in on.
If my link specified "mypage.html" why on earth would a spider look for any other extension. htm, asp, cgi whatever.