A system and method is provided for reliably detecting the file type of a client-requested and by-passing conventional ICAP processing if the detected file type corresponds to a non-viral file. The client-requested file is examined to determine whether it contains a predefined byte pattern (or "file signature") corresponding to a non-viral type. The signature may be embodied as one or more predetermined "magic bytes" located at known offsets. For instance, the client-requested may be identified as a particular type of image file if it contains the set of magic bytes associated with that image format. Unlike prior implementations, when the client-requested file is determined to contain magic bytes corresponding to a non-viral file type, such as an image file, the file is returned to the requesting client without performing conventional ICAP virus-scanning operations.