A CIFTI-2 file name states the file type (.dscalar, .pdconn, ...), and
the type decides which matrix index types the two dimensions have. A file whose name
contradicts its content is silently misread by other software, so this is an error.
cifti.validate.file.extension(filepath, file_type)NULL, invisibly. Stops if the file name names a different file type.
character string, the file name to write.
a one row data.frame, see cifti.file.type.for.axes.