A Uniform Type Identifier (UTI) is a text string used by Apple Inc. software to uniquely identify data types such as document or image files, folders, and application bundles. Introduced in Mac OS X 10.4, UTIs integrate with Spotlight for categorizing content. They replace unreliable methods like MIME types or filename extensions. UTIs use a reverse-DNS naming structure with permitted ASCII and Unicode characters, excluding colons and slashes for compatibility. Supporting multiple inheritance, UTIs allow a file to have multiple relevant types, and are case-insensitive to ensure consistency across the system.
Background
One of the difficulties in maintaining a user-accessible operating system is establishing connections between data types and the applications or processes that can effectively use such data. For example, a file that contains picture data in a particular compression format can only be opened and processed in applications that are capable of handling picture data, and those applications must be able to identify which compression type was used in order to extract and work with that data. In early computer systems – particularly DOS, its variants, and some versions of Windows – file associations are maintained by file extensions. The three to four character code following a file name instructs the system to open the file in particular applications.
Beginning with System 1,4 Macintosh operating systems have attached type codes and creator codes as part of the file metadata. These four-character codes were designed to specify both the application that created the file (the creator code) and the specific type of the file (the type code) so that other applications could easily open and process the file data. However, while type and creator codes extended the flexibility of the system — a particular type of file was not restricted to opening in a particular application — they suffered many of the same problems as file extensions. Type and creator codes could be lost when files were transferred across non-Macintosh systems (such as Unix-based servers), and the plethora of type codes made identification problematic.
In addition, the classic Mac OS did not recognize file extensions at all, leading to unrecognized file errors when files were transferred from DOS/Windows systems. OPENSTEP, which formed the basis of Mac OS X, used extensions, and early versions of Mac OS X followed suit. This led to some controversy with users and developers coming to OS X from NeXT or Windows origins advocating for continued use of file extensions, and those coming from Classic Mac OS urging Apple to replace or supplement file extensions with type and creators.5
Other file identification types exist: for example, MIME types are used for identifying data that is transferred over the web. However, Apple's UTI system was designed to create a flexible file association system that would describe data hierarchically and allow for better categorization and searching, standardize data descriptions across contexts, and provide a uniform method of expanding data types. For instance, the public.jpeg and public.png UTIs inherit from the public.image UTI, allowing users to search narrowly for JPEG images or PNG images or broadly for any kind of image merely by changing the specificity of the UTI used in the search. Further, application developers who design new data types can easily extend the UTIs available. For example, a new image format developed by a company may have a UTI of com.company.proprietary-image and be specified to inherit from the public.image type.
Apple's macOS continues to support other forms of file association, and contains utilities for translating between them, but will use UTIs by preference where available.
UTI structure
Apple maintains the public.* domain as a set base data types for all UTIs. Other UTIs are associated with these base UTIs by conformance, a system similar to class inheritance. UTIs that conform to other UTIs share a basic types, and in general any application that works with data of a more general UTI should be able to work with data of any UTI that conforms to that general UTI.
Apple public UTIs
The most basic public UTIs in the Apple hierarchy are as follows:
| Identifier | Conforms to | Comment |
|---|---|---|
| public.item | base class in the physical hierarchy | |
| public.content | base class for all document content | |
| public.data | public.item | base class for all files, byte streams, pasteboard, etc. |
| public.image | public.data, public.content | base class for all images |
UTIs are even used to identify other file type identifiers:
| Identifier | Conforms to | Comment |
|---|---|---|
| public.filename-extension | public.case-insensitive-text | Filename extension |
| public.mime-type | public.case-insensitive-text | MIME type |
| com.apple.ostype | public.text | Four-character code (type OSType) |
| com.apple.nspboard-type | public.text | NSPasteboard type |
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."6
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 macOS 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 functions, such as enabling Quick Look for new file types.
List of common third-party UTIs
| Description | UTI | Extensions | Conforms to | MIME types | Ref. |
|---|---|---|---|---|---|
| OPML document | org.opml.opml | .opml | public.xml | text/xml, text/x-opml, application/xml | 7 |
| Markdown document | net.daringfireball.markdown8 | .md, .markdown | public.plain-text | text/markdown | 9 |
| POSIX Paths document | cc.utis.paths-file | .paths | public.utf8-plain-text | not defined | 10 |
| Pasteboard Types | org.nspasteboard.TransientType, org.nspasteboard.ConcealedType, org.nspasteboard.AutoGeneratedType, org.nspasteboard.source | not a file type | — | — | 11 |
Looking up a UTI
To get the UTI of a given file, use the mdls (meta data list, part of Spotlight) command in the Terminal.
mdls -name kMDItemContentType -name kMDItemContentTypeTree -name kMDItemKind FILE
References
"Uniform Type Identifiers Overview". Guides and Sample Code. Apple Inc. October 29, 2007. Retrieved September 12, 2016. https://developer.apple.com/library/prerelease/content/documentation/FileManagement/Conceptual/understanding_utis/understand_utis_intro/understand_utis_intro.html ↩
"Uniform Type Identifiers Overview". Guides and Sample Code. Apple Inc. October 29, 2007. Retrieved September 12, 2016. https://developer.apple.com/library/prerelease/content/documentation/FileManagement/Conceptual/understanding_utis/understand_utis_intro/understand_utis_intro.html ↩
"Uniform Type Identifiers — a reintroduction - Tech Talks - Videos". Apple Developer. Retrieved May 17, 2022. https://developer.apple.com/videos/play/tech-talks/10696 ↩
"Folklore.org: The Grand Unified Model (2) - The Finder". www.folklore.org. Retrieved April 12, 2018. http://www.folklore.org/StoryView.py?project=Macintosh&story=The_Grand_Unified_Model_The_Finder.txt ↩
"Mac OS X 10.1 File Name Extension Guidelines - Cocoabuilder". www.cocoabuilder.com. Retrieved April 12, 2018. http://www.cocoabuilder.com/archive/cocoa/38552-mac-os-10-1-file-name-extension-guidelines.html ↩
"Uniform Type Identifiers Overview". Guides and Sample Code. Apple Inc. October 29, 2007. Retrieved September 12, 2016. https://developer.apple.com/library/prerelease/content/documentation/FileManagement/Conceptual/understanding_utis/understand_utis_intro/understand_utis_intro.html ↩
OPML 2.0 spec http://dev.opml.org/spec2.html ↩
"Uniform Type Identifier For Markdown". Daring Fireball. Retrieved August 21, 2019. https://daringfireball.net/linked/2011/08/05/markdown-uti ↩
"Markdown". Daring Fireball. https://daringfireball.net/projects/markdown/ ↩
DotPathsFileSpec on GitHub https://github.com/utiscc/DotPathsFileSpec ↩
Identifying and Handling Transient or Special Data on the Clipboard http://nspasteboard.org ↩