Just as system sector viruses can remain resident in memory and use stealth techniques to hide their presence, file viruses can also hide this way. If you do a directory listing, you will not see any increase in the length of the file and if you attempt to read the file, the virus will intercept the request and return your original uninfected program to you. Overlay files have various extensions, but .OVR and .OVL are common. Indeed, as operating systems become more advanced, typically more files become able to contain executable code and thus be vulnerable to infection. Summary File viruses number in the thousands, but are not the most widely found in the wild.
File viruses have a wide variety of infection techniques and infect a large number of file types.