An associated file is the mime.types file, which associates filename extensions with a "MIME type". If the MIME type is properly set, this is unnecessary, but MIME types may be incorrectly set, or set to a generic type such as , and mime.types allows one to fall back on the extension in these cases. Similarly, since many file systems do not store MIME type information, but instead rely on the filename extension, a mime.types file is frequently used by web servers to determine MIME type.