Development:Using the File API in Moodle forms

In Moodle 2.o onward, we introduced Repository API to fetch files from external sources, Repository API will move to files into draft areas, then modules will process moving draft area files into a proper fileareas, this documentation will demonstrate how to do that.

There are three form elements involved with Repository API, they are file manager, file picker and editor, the legacy upload button will be replaced by file picker or file manager, file picker and file manager work similar, the only difference is file manager can fetch multiple files, file picker only fetch one file. Editor element is introduced to replace legacy htmleditor element, I will talk about them respectively.