head property
the first bytes of the file. @Mimes and @Image read the real
file type from them, and @Dimensions reads the image size.
a file in memory gives all its bytes (no copy). a file on disk gives
its first 128 KB, so the size of a JPEG after its photo metadata is
still found.
Implementation
@override
List<int> get head {
return _head ?? _bytes ?? const <int>[];
}