In the body: air attenuates X-rays a little; fat attenuates X-rays more than air but less than bone attenuates X-rays the most.
These differences in attenuation result in differences in the level of exposure of the film. When the photographic film is developed, bone appears white on the film because this region of the film has been exposed to the least amount of X-rays. Air appears dark on the film because these regions were exposed to the greatest number of X-rays.