As per Trickie, and to expand, it's a convention (like the .sh thing) that if an entry under /etc ends in '.d' then it's a actually a dir, not a file.
Strictly speaking, under *nix, file extensions are a matter of (optional) convention, but because almost everybody uses them (and some tools recognise certain ones by default), it's as well to keep using them.
You may find these useful:
When in doubt, use the file cmd http://linux.die.net/man/1/file
before trying to open/read a file.
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