It turned out that there is some difference between MultipartHttpServletRequest implementation in Grails 1 and Grails 3 and this is probably why it couldn't cast this class now.

Since CommonsMultipartFile is an implementation of MultipartFile interface:

public class CommonsMultipartFile implements MultipartFile, Serializable {

we can (and should) use it as a type here, since it is even more correct in terms of dependency inversion principle, and the error disappears and the import works correctly.