Write Image to GitRepo

I am trying to upload an Image to GitHub from the browser. I download the image into a blob, convert the blob into base64 and the use the writeFile method. However, the data never arrives correctly in GitHub.
```JS
fetch(url).then(response => response.blob().then(blob => {
let reader = new FileReader();
reader.onloadend = () => {
var base64result = reader.result.substr(reader.result.indexOf(',') + 1);
let fileName = this.getFileName(image.url)
repository.writeFile(this.github.branch, fileName, base64result, 'Quant-UX Export').then(result => {
resolve(result)
})
}
reader.readAsDataURL(blob);
}))
```

Start by tracing the repository.writeFile call in the browser wrapper and compare its handling of the base64result produced by FileReader. Reproduce the image upload with the shown fetch and writeFile sequence; done means the uploaded image arrives correctly in GitHub.

索引モデルが issue の本文から書いたものです。