Uniform Type Identifier

A Uniform Type Identifier (UTI) is a string defined by Apple Inc. that uniquely identifies the type of a class of items. Added in Apple's Mac OS X 10.4 operating system, UTIs are used to identify the type of files and folders, clipboard data, bundles, aliases and symlinks, and streaming data. Mac OS X's desktop search technology, Spotlight, uses UTIs to categorize documents.^[1] One of the primary design goals of UTIs was to eliminate the ambiguities and problems associated with inferring a file's content from its MIME type, filename extension, or type or creator code.^[1][2]

UTIs use a reverse-DNS structure. UTIs support multiple inheritance, allowing multimedia files to be identified as not as single type (as in MIME), but as all the types it is; an identifier can inherit from public.audio, public.video, public.text, public.image, etc.^[1] UTIs are stored as Core Foundation strings; allowable characters are A–Z, a–z, 0–9, "-", ".", and all Unicode characters above U+007F.^[2]

The public.* domain is only editable by Apple and contains the base data types used by all UTIs.^[1]

UTIs are even used to identify other file type identifiers:^[1]

Dynamic UTIs can be created as needed by applications; these have the prefix dyn. and take the form of "a UTI-compatible wrapper around an otherwise unknown filename extension, MIME type, OSType, and so on."^[2]

Third-Party UTIs

Apple provides a large collection of system-declared Uniform Type Identifiers. Third-party applications can add UTIs to the database maintained by Mac OS X by "exporting" UTIs declared within the application package. Because new UTIs can be declared to "conform to" existing system UTIs, and declarations can associate the new UTIs with file extensions, an exported declaration alone can provide the operating system with enough information to enable new functionality, such as enabling Quick Look for new file types.

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