file/add - Creates a new file.

Method parameters:
name - The name of the file.
file - The content file to upload.
parent_id - The ID of the parent folder.
auto_rename - Give the file a unique name if a file with the same name already exists.

Returns a JSON document of the following format:
<number> - The ID of the newly created file.

Expected errors:
ENTER_NAME - The name is empty.
FILE_ALREADY_EXISTS - A file with the name already exists.
PARENT_FOLDER_NOT_FOUND - A parent folder with the ID doesn't exist.
SELECT_FILE - The content file is empty.
FILE_ERROR - An error occured while uploading the file.