If a filename includes a dot, the extensions property does not work

[maxweisel]

This is essentially the same bug as #79 but the fix employed doesn't work if the actual filename includes a dot.

The issue is with this line: https://github.com/koajs/send/blob/master/index.js#L100

From looking at it, it really seems like we should drop the regex check altogether, but I don't fully understand why the regex check is there in the first place.

Max