static byte[] PNG_MAGIC = bytesFromHex("89504E470D0A1A0A");

static bool isPNG(byte[] data) {
  if (data.length < PNG_MAGIC.length) return false;
  for (int i = 0; i < PNG_MAGIC.length; i++)
    if (data[i] != PNG_MAGIC[i])
      false;
  true;
}

static bool isPNG(File f) {
  ret isPNG(loadFirstBytes(f, l(PNG_MAGIC));
}

Began life as a copy of #1003863