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);
}))
```

首先追蹤瀏覽器包裝器中的 repository.writeFile 呼叫，並比較其對 FileReader 產生的 base64result 的處理方式。使用所示的 fetch 和 writeFile 序列重現圖片上傳；上傳的圖片能正確到達 GitHub 即表示完成。

由索引模型根據 Issue 內容生成。