A file with no extension is a computer file whose name doesn't end with a period followed by a few letters (like .txt or .jpg). File extensions normally signal the file's format and which program should open it. Without one, the operating system relies on other methods to determine the file type, such as inspecting the file's internal data structure (metadata or 'magic bytes') or referring to hidden attributes.

For example, many system files on Unix-like operating systems (Linux, macOS) lack extensions, such as common configuration files stored in the /etc/ directory or logs generated by system services. Source code files, like a README file in a software project repository, also frequently omit extensions.

The main advantage of omitting an extension can be tidiness or convention within specific technical environments. However, it significantly hinders usability for end-users, as the operating system and users struggle to identify the file's purpose or compatible applications without examining the internal content. While reliance on internal metadata is robust, standardization for handling extensionless files remains inconsistent across platforms, potentially causing confusion outside specific server or programming contexts.