Notes and attachment in Winter'18 release
In winter'18 Notes & Attachments related list will no longer have an Upload File button. In my application, I am inserting the attachment using the apex code. I just want to confirm if we can still upload attachments using apex or not.
What's you're seeing in the winter 17 Release notes is a UI issue, not a functionality issue in terms of the platform for the attachments. You should still be able to create the custom functionality.
Please refer to Knowledge Article,
https://help.salesforce.com/articleView?id=New-Note-button-Notes-Attachments-related-list&language=en_US&type=1 which says that the "Upload Files" button will remain available in the legacy Notes & Attachments related list.