This directory contains part of the browser side implementation of various filesystem related APIs.
//storage/browser/fileapi/ contains the rest of the browser side implementation, while blink/renderer/modules/filesystem contains the renderer side implementation and blink/public/mojom/filesystem contains the mojom interfaces for these APIs.
FileSystemManagerImpl is the main entry point for calls from the renderer, it mostly redirects incoming mojom calls to a storage::FileSystemContext instance.
FileSystemChooser uses ui::SelectFileDialog to show a file or directory picker, and is responsible for granting a process the right permissions for actually accessing the files that were selected.