Filename extension

A filename extension is a suffix, usually following a period, that conventionally indicates a file’s format or intended application. It is a naming convention, not a guarantee of a file’s contents.

File association: A rule linking a filename pattern or file type to an application that opens or handles it. Operating systems often use extensions to choose which application opens a file.
File signature: A byte pattern used to identify a file format, often found at the beginning of a file. A signature can reveal a format even when the filename extension is absent or incorrect.
Open With: A user-interface command for choosing an application to open a file. It can override the application normally associated with a file’s extension.
File system: A system for organizing, storing, and retrieving files and directories on digital storage. Filesystems store filenames, though their rules for naming and interpreting extensions vary.
MIME type: A standardized label identifying the media type and subtype of digital content. MIME types identify formats by metadata rather than relying only on filename suffixes.
Content type: Metadata describing the media format of a message body or digital resource. Network protocols can convey format information independently of a filename extension.
Executable file: A file containing instructions that a computer can run as a program. Executable suffixes can influence whether a system treats a file as runnable.
Dot (punctuation): A small punctuation mark used in writing and notation, also called a full stop or period. A period commonly separates a filename’s stem from its extension.
Magic number (programming): A fixed byte sequence near the start of a file that helps identify its format. File-inspection tools can use these bytes to detect formats when extensions mislead.
File format: A specification defining how digital information is encoded and organized in a file. The extension names or suggests a format; the format itself defines the data structure.

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