Get MIME type of File object

I have a dart:html File object and I want to determine the MIME type from the file contents rather than the extension.

You can use the mime package's function lookupMimeType and pass in the headerBytes by calling slice on the File with defaultMagicNumbersMaxLength

import 'dart:async';
import 'dart:html';
import 'package:mime/mime.dart' as mime;

Future<String?> getMimeType(File file) async {
  // Create a slice for the header.
  final slice = file.slice(0, mime.defaultMagicNumbersMaxLength);

  // Read the file header's contents.
  final fileReader = FileReader();
  fileReader.readAsArrayBuffer(slice);
  await fileReader.onLoad.first;
  final header = fileReader.result as List<int>;

  // Empty string for the file name because it's not relevant.
  return mime.lookupMimeType('', headerBytes: header);
}