a-image，预览图片时，可支持加载原图的功能

[imbossfirst]

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What problem does this feature solve?

图片预览时，在工具栏添加查看原图的功能，开发者提供图片地址，如果地址存在且合理，那么需要获取文件大小，【原图**（MB）】，点击原图的按钮，给出下载进度，并且下载可取消，下载完成后替换预览的图片隐藏原图按钮

What does the proposed API look like?

添加字段source