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?

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