SPFile "Content Type ID" is default even after changing programmatically when excel has macros

Question

Below is the code used to update the content type of an spfile.

newFile.Item["Content Type ID"] = spWeb.ContentTypes["Subject Attachment"].Id; newFile.Item.Update();

but "Content Type ID" for the attachment file is assigned default value when the excel file has the macro.

How do i change the code such that it does not take the default "Content Type ID" when excel file has macro.

NOTE: This behavior is observed only when attachment file has macro.