Currently, only one FileSystemSyncAccessHandle may be open at a time per file, preventing an origin from reading and writing to the same file from multiple tabs easily. Conversely, multiple FileSystemWritableFileStreams can be simultaneously open, letting multiple writers clobber each other. Introducing new locking modes for FileSystemSyncAccessHandle and FileSystemWritableFileStream allows opening either multiple readers/writers or an exclusive writer to a file entry, depending on the application's use case.