[feature] Remove ext mandatory property for saveAs

[EArminjon]

import 'package:file_saver/file_saver.dart';
import 'package:mime/mime.dart';
import 'package:path/path.dart';

await FileSaver.instance.saveAs(
        filePath: filePath,
        name: basenameWithoutExtension(filePath),
        mimeType: MimeType.custom,
        customMimeType: lookupMimeType(filePath),
        ext: extensionFromMime(
          lookupMimeType(filePath)!,
        ) ?? '',
      ),

If file didn't have any extension, as this lib force us to add one, we got an extra dot at the end of the file. It's a possible scenario to have file without any extension in names.