根据原图，计算一个宽或高最大100的图片的计算方法

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const previewImageRatio = previewImageWidth / previewImageHeight; // 原图宽高比例                        let thumbnailHeight,thumbnailWidth;// 缩略图高和宽                        if(previewImageRatio > 1){                            thumbnailWidth = 100; // 缩略图宽度固定为 100                            thumbnailHeight = thumbnailWidth / previewImageRatio;                        }else{                            thumbnailHeight = 100;                            thumbnailWidth = thumbnailHeight * previewImageRatio;                        }