It seems that Creator 15 added a new mime type "application/cu-seeme" with the "*.cu" file pattern (file ending). This is the same as is often used for cuda files. The new mime type tells creator to open the file with the binary editor, which is not so nice for coding. And I think that it's more probable that developers will have to open cuda code files compared to cu-seeme files.

A workaround is to change the mime type pattern of cu-seeme to something like "*.cu_blah".