filtyp

Filtyp, or file type, is a classification that indicates an electronic file's format and, often, its intended use. The concept is central to how software reads, displays, or executes the contents of a file. Common indicators include the filename extension (such as .txt, .jpg, .pdf), the MIME type assigned by software or web servers (text/plain, image/jpeg, application/pdf), and, for greater reliability, the file's internal signature or header bytes (magic numbers).

Determination of a file type can rely on different signals. Extensions are convenient but can be wrong or misleading. MIME types help applications decide how to handle a file when it is downloaded or opened from a network. Some file formats are containers, such as .docx or .jar, which package multiple components inside a single file.

Usage: Operating systems and applications map file types to programs that can open or process them, and users can customize these associations. Web browsers rely on MIME types to determine whether to display content inline, download it, or pass it to a plug-in.

Security considerations: Mislabelled extensions can conceal executable code or scripts. Relying solely on extensions is risky; many systems apply additional checks, such as content sniffing and sandboxing, before allowing actions on a file.

Examples: image/jpeg (.jpg, .jpeg); image/png (.png); application/pdf (.pdf); text/plain (.txt); application/zip (.zip); for Microsoft Word, .docx is a ZIP-based container containing OpenXML documents.

See also: File extension, MIME type, Magic number, File format identification.