How to recognize a JPEG or a GIF image when the file has no extension?

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Hi!

I download images which have no extension, with a System.Net.WebClient.
I would like to save them with a different name and with the extension
..jpg if they really are of JPEG type. Because sometimes I could receive
GIF images, and I would like to do the same.

I tried to look for the image type in the WebClient object, but I
didn't find anything.
And once the file has been saved without an extension on my computer,
when I right-click on it, and then Properties and Summary, Windows
seems to know that it is a JPEG or a GIF file.

I read on the internet that these informations are contained in the
IPTC datas, but I have no idea on how to get them from the file.

Any ideas for that would be welcome.

Thx in advance.
It is pretty much a hurry!!

Clement

 

[Gabriele G. Ponti]

Read the first few bytes of the files, and check for a valid header.

GIF header information
http://www.onicos.com/staff/iz/formats/gif.html

JPEG header information
http://www.obrador.com/essentialjpeg/HeaderInfo.htm