Dropbox iOS API缩略图下载会出错

Question

我尝试使用Dropbox API下载缩略图，并且我使用缩略图获取了每个图像的错误：

[WARNING] DropboxSDK: error making request to /1/thumbnails/dropbox(null)

ERROR LOADING THUMBNAIL: The operation couldn’t be completed. (dropbox.com error 400.)

我调用了以下方法来加载我的缩略图：

[[self restClient] loadThumbnail:obj.url ofSize:@"medium" intoPath:obj.path];

我的代表缩略图：

- (void)restClient:(DBRestClient*)client loadThumbnailFailedWithError:(NSError*)error {
NSLog(@"ERROR LOADING THUMBNAIL: %@", [error localizedDescription]);
}

- (void)restClient:(DBRestClient*)client loadedThumbnail:(NSString*)destPath {
NSLog(@"Loaded: %@", destPath);
UIImage *image = [UIImage imageWithContentsOfFile:destPath];
//Do something with my thumbnail
}

我做错了什么？