Firebase triggered function uploads images to storage that can not be previewed in console

Question

With the following code I resize and upload every new image to the storage bucket.

This works, as in the files are created and uploaded to storage I can see the correctly named files. BUT those files seem not to be able to previewed inside the console. (security?)

exports.generateThumbs = functions
  .region("europe-west1")
  .storage.object()
  .onFinalize(async (object) => {
    const bucket = admin.storage().bucket(object.bucket);
    const filePath = object.name;
    const fileName = filePath.split("/").pop();
    const bucketDir = path.dirname(filePath);

    const workingDir = path.join(os.tmpdir(), "resize");
    const tmpFilePath = path.join(workingDir, "source.jpeg");

    if (
      fileName.includes("_resized_") ||
      !object.contentType.includes("image")
    ) {
      console.log("exiting function");
      return false;
    }

    // 1. Ensure thumbnail dir exists
    await fs.ensureDir(workingDir);

    // 2. Download Source File
    await bucket.file(filePath).download({
      destination: tmpFilePath,
    });

    // 3. Resize the images and define an array of upload promises
    const sizes = { thumb: 100, small: 350, medium: 1000 };

    const uploadPromises = Object.keys(sizes).map(async (k) => {
      const size = sizes[k];
      const imgName = `${fileName}_resized_${k}`;
      const thumbPath = path.join(workingDir, imgName);

      // Resize source image
      await sharp(tmpFilePath)
        .resize(size, size, { fit: "inside", withoutEnlargement: true })
        .toFile(thumbPath);

      // Upload to GCS
      return bucket.upload(thumbPath, {
        destination: path.join(bucketDir, imgName),
        uploadType: "media",
        metadata: {
          contentType: object.contentType,
        },
      });
    });

    // 4. Run the upload operations
    await Promise.all(uploadPromises);

    // 5. Cleanup remove the tmp/thumbs from the filesystem
    return fs.remove(workingDir);
  });

original uploaded file can be previewed

the files generated by the function can not be previewed

I have no clue what is going on, and I can not seem to fix it after hours of trying.

Anyone has an idea why these files are uploaded, have a file size, have the correct name (_thumb, _medium, etc..) but can not be viewed inside the console.

Thank you for any feedback,

Matthias