Hi everyone, the problem is not with the version 2.1.1, the problem is with versions smaller then 2.1.1. Because if you can import any extension with every objects with it, it will be a security problem. You can refer to the guide i attached, which i downloaded today. Please refer to this link for explanations... Have a good day
Further to this, I was experiencing the same issues and deleting those 'extra' files works. You should also delete anything like a LICENSE file as I found that some extensions have these and it causes the import to fail.
Yes, it varies from extension to extension, depending what unsupported files the zip contains.
Read the article that kerem cited, but please note the conditions that Stefan mentions are conditions for failure, not for success. To clean up your zip, make sure it does not contain any of the files mentioned.