filetype

Filetype, or file type, refers to the classification that describes the format, encoding, and structure of a file. It informs software how to interpret the data and how to present it to the user.

The most familiar indicators are file extensions, the suffix such as .txt or .jpg, and MIME types used by internet protocols (for example text/plain or image/jpeg). When extensions are missing or misleading, systems may rely on content signatures called magic numbers to identify a file.

A single file type describes a format, but a file may be stored in different containers or encodings, and a single container can hold data in multiple formats. For example, a PDF file has a fixed format, while a ZIP container can wrap many file types.

Operating systems use filetype information to determine default applications and handling rules. Web servers and email clients rely on MIME types to indicate how to treat attachments and responses, guiding browsers to display or download content appropriately.

Some tools identify filetypes by content alone, notably the Unix command line program 'file', which analyzes file headers and data to guess the type with varying accuracy.

Mislabeling or extension spoofing can lead to security issues, including incorrect handling or execution of malicious content. Best practices favor consistent use of extension, MIME type, and content checks to reduce risk.