[1] Colons and slashes are prohibited for compatibility with Macintosh and POSIX file path conventions.
UTIs support multiple inheritance, allowing files to be identified with any number of relevant types, as appropriate to the contained data.
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.
Further, application developers who design new data types can easily extend the UTIs available.
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.
and take the form of "a UTI-compatible wrapper around an otherwise unknown filename extension, MIME type, OSType, and so on.
To get the UTI of a given file, use the mdls (meta data list, part of Spotlight) command in the Terminal.