Many systems, including DOS and UNIX, allow a filename extension that consists of one or more characters following the proper filename. For example, image files are usually stored as .bmp, .gif, .jpg or .tiff. Audio files are often stored as .aud or .wav. There are a multitude of file extensions identifying file formats. The filename extension should indicate what type of file it is; however, users may change filename extensions to evade firewall restrictions or for other reasons. Therefore, file types should be identified at a binary level rather than relying on file extensions. To research file types, see ( Different applications can often recognize only a predetermined selection of file types. See also Format..”