previewThumbnailSize property
Preview thumbnail size in the viewer. 预览时图片的缩略图大小
This only works on images and videos since other types does not have to request for the thumbnail data. The preview can speed up by reducing it. 该参数仅生效于图片和视频类型的资源，因为其他资源不需要请求缩略图数据。 预览图片的速度可以通过适当降低它的数值来提升。
Default is null, which will request the origin data.
默认为空，即读取原图。
Implementation
final ThumbnailSize? previewThumbnailSize;