Checks the 'upload_path' option, which should be from the web root folder, and if it isn't empty it will be used. If it is empty, then the path will be 'WP_CONTENT_DIR/uploads'. If the 'UPLOADS' constant is defined, then it will override the 'upload_path' option and 'WP_CONTENT_DIR/uploads' path.
The upload URL path is set either by the 'upload_url_path' option or by using the 'WP_CONTENT_URL' constant and appending '/uploads' to the path.
If the 'uploads_use_yearmonth_folders' is set to true (checkbox if checked in the administration settings panel), then the time will be used. The format will be year first and then month.
If the path couldn't be created, then an error will be returned with the key 'error' containing the error message. The error suggests that the parent directory is not writable by the server.
On success, the returned array will have many indices: 'path' - base directory and sub directory or full path to upload directory. 'url' - base url and sub directory or absolute URL to upload directory. 'subdir' - sub directory if uploads use year/month folders option is on. 'basedir' - path without subdir. 'baseurl' - URL path without subdir. 'error' - false or error message.