add-type in clipboard-client%

Adds a new data format name to the list supported by the clipboard client.

• (send a-clipboard-client add-type format) -> void
     format : string

  The format string is typically four capital letters. (Under Mac OS, only four characters for format are ever used.) For example, "TEXT" is the name of the simple text format. New format names can be used to communicate application- and platform-specific data formats.