Hi Hanh,
The GetFile.aspx.cs file has been removed in Kentico 10. Instead, a mapped handler is used to handle these requests. The implementation of the handler is in the CMS.DocumentEngine.GetAttachmentHandler class. However, you would need a source code license to be able to change its code.
Depending on your needs, you may create a handler for the RequestEvents.End.Execute event and implement your custom logic there. To check if the request is being processed by GetAttachmentHandler, you can use the following condition in the handler:
if(CMSHttpContext.Current.Handler is IGetAttachmentHandler) { do something }
I hope it helps!