A New Discovery?: Omitting File Extension in SRC attribute of IMG tag

[First of all, I don't know whether I have selected a proper section for this or not. I didn't find a perfect section for this. If the selected section for this trick is not valid, I am sorry; and if possible, I would change it if you have a good section suggestion.] I accidently found a new thing that the extension (including the dot) of an image file can be omitted in the SRC attribute of IMG tag in HTML file. So

<img src="pencil" />

is valid and showing the image if the actual file name is "pencil.jpg". When I put 3 images pencil.png, pencil.jpg and pencil.gif in a folder and put 3 IMG tags in HTML file.

  <img src="pencil" />
  <img src="pencil" />
  <img src="pencil" />

----- Update: This code is showing 3 PNG images in Firefox and Internet Explorer; but Chrome, Opera and Safari are showing 3 JPEG images! ----- Omitting file extension also worked when I used stylesheet:

It is displaying the image as background in a 100x100 pixel sized Div. This trick worked on my computer on WAMP server (Apache) (http://localhost/example.htm), but it didn't work when I opened the file directly in browser (file:///c:/wamp/www/example.htm). This trick also worked with HTML-5 document. I have Win 7 Ultimate 32-bit with WAMP Server 2.0c installed. ----- Update: I checked this on both Windows and Linux servers but it is not working on live servers. :-( Totally confused. It is working on my computer having server of Apache and Windows 7 OS but it is not working on live Windows or Linux servers! It is also not working when the file is opened from local harddisk. -----