Tweak JSON attachment parsing for API

    * Don't require a content-type, so data:,this is content
      is perfectly allowable. Such content is assumed text/plain
    * Support the charset hint, so
      data:text/plain;charset=iso-8859-1,content here
      will be translated to UTF-8 if the php iconv() function exists.
      Otherwise, content is left intact and assumed by the database to by
      UTF-8 already