In the Alibaba Cloud CDN + OSS mode, the files are still directly downloaded during access to URL. The reason is that, when the default third-level domain name is accessed, the attachment attribute is added to the files according to the OSS policy, so that the files are forcibly downloaded.
Check the configuration of Back-to-source host on Alibaba Cloud CDN. Back-to-source host on Alibaba Cloud CDN should be an accelerated domain name, instead of a bucket domain name (that is, not the default OSS domain name ended with aliyuncs.com). Generally, the problem can be solved by modifying the configuration of the back-to-source host.
If the problem persists, contact Alibaba Cloud After-Sales Technical Support.