Adds default content types to [Content_Types].xml.
Word refuses to open a package that contains a part whose extension it has no content type for, so every media file added at render time registers one.
Summary
Functions
Registers a default content type for extension, ignoring duplicates.
Functions
@spec add_content_type(Sablon.XML.Document.t(), binary(), binary()) :: :ok
Registers a default content type for extension, ignoring duplicates.