根据原图，计算一个宽或高最大100的图片的计算方法

 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;
                        }