default @NonNls @Nullable String getCharset(@NotNull VirtualFile file, byte @NotNull [] content) {
    // TODO see MetadataJsonFileType (it's actually text but tries indexing itself as binary)
    // if (isBinary()) {
    //   throw new UnsupportedOperationException();
    // }
    return null;
  }
default @NonNls @Nullable String getCharset(@NotNull VirtualFile file, byte @NotNull [] content)