Adobe Universal Container Format
From Just Solve the File Format Problem
The Universal Container Format (UCF) was developed by Adobe for use in PDFXML. It is based on the Open Container Format (OCF) used in EPUB, itself based on OpenDocument. It is a subset of ZIP, specifying a mandatory initial uncompressed mimetype file, an optional META-INF folder containing XML files, and other restrictions on file names and compression methods.
It can contain XMP metadata in the file META-INF/metadata.xml.
Some formats include +zip at the end of the mimetype file's contents, and some do not.
