When using fetch, the content type of the blob is not set. As an example you can run the sample below.
b.type is going to be an empty string.

fetch('https://picsum.photos/200')
        .then(res => res.blob())
        .then(b => {
            console.log('Content type:', b.type);
        })
        .catch(console.error)

That's because in the fetch polyfill (/src/js/polyfills/fetch/body.js) the blob is not set with the type option.

The fix would be to just take the Content-Type header value and set it as the type.