downloading only the header of a file

I'm using mechanize for scraping a website which works nicely, however since you can't tell from a 开发者_开发百科link what kind of file it is linking to e.g. http://somesite.com/images.php?get=123 is it possible to download only the header only?

It doesn't have to use mechanize but is there any Rails way of doing this?

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Mechanize has the headers available after downloading a file. page.header['xxx'] or file.header['xxx'].