Mapping Part Content Type
Part content types are used for associating content types with part data within a package. In ZIP archives, content type information is stored using the common mapping pattern that stores this information in a single XML stream as follows:
Package implementers shall store content type data in an item(s) mapped to the logical item name with the prefix_name equal to “/[Content_Types].xml” or in the interleaved case to the complete sequence of logical item names with that prefix_name. [M3.10]
Package implementers shall not map logical item name(s) mapped to the Content Types stream in a ZIP archive to a part name. [M3.11]
Note: : Bracket characters "[" and "]" were chosen for the Content Types stream name specifically because these characters violate the part naming grammar, thus reinforcing this requirement.