copy-file behavior

It seems strange to me that copy-file adds a file extension
to the destination:

[1]> (copy-file "/home/don/2014.05.18.rr.sep.xE.xls" "/tmp/foo")
((#P"/home/don/2014.05.18.rr.sep.xE.xls" #P"/tmp/foo.xls" 75938))
[2]> 

And, not surprisingly, there is no file named /tmp/foo now, but
there is one named /tmp/foo.xls instead.

This isn't mentioned in impnotes.
Is it intended?  What's the justification?
How can I get the destination I asked for?

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