AWS为S3存储桶中的资产预先分配了URL

We upload contents to S3 private bucket. After uploading contents we access them through presigned url. MP4, images are working fine when we access that url through the browser. But when we try to access SWFs and PDFs, browser prompts to download content. And also it won't happen when we try to access assets from public bucket.We upload contents to S3 private bucket. After