This files describes API changes in /dataformat/ download system,
information provided here is intended especially for developers.

=== 3.9.8 ===
* Optional $callback for the following \core\dataformat methods now receive a second argument to define
  whether the current format writer supports HTML:
  - download_data()
  - write_data()
  - write_data_to_filearea()

=== 3.9 ===
* The following methods have been added to the base dataformat class to allow instances to export to a local
  file. They can be overridden in extending classes to define how files should be created:
  - start_output_to_file()
  - close_output_to_file()
* Calls to the following dataformat plugin methods have been removed:
  - write_header()
  - write_footer()
* The following methods have been added to the base class to allow instances to define support for exporting
  HTML content, with additional support for defining how images should be embedded:
  - supports_html()
  - export_html_image_source()
* Dataformat writers should also call the following method to ensure data is properly formatted before being
  written, which takes into account prior methods defining support for HTML:
  - format_record()

=== 3.4 ===
* In order to allow multiple sheets in an exported file the functions write_header() and write_footer() have
  been removed from core dataformat plugins and have been replaced.
  - write_header() has been replaced with the two functions start_output() and start_sheet().
  - write_footer() has been replaced with the two functions close_output() and close_sheet().
  For backwards compatibility write_header() and write_footer() will continue to work but if used will
  trigger the function error_log().

=== 3.1 ===
* Added new plugin system with low memory support for csv, ods, xls and json