The [Content_Types].xml manifest that lives at the root of every OPC
package.
It maps file extensions (Default) and individual part paths (Override) to
MIME-like content type strings. ExVEx preserves every entry verbatim so
unknown content types (custom XML parts, plugin data, future schema
extensions) survive a read/edit/save cycle.
Summary
Types
@type t() :: %ExVEx.Packaging.ContentTypes{ defaults: [ExVEx.Packaging.ContentTypes.Default.t()], overrides: [ExVEx.Packaging.ContentTypes.Override.t()] }