Matlab Answers forum - cant attach slx but can attach mdl

Is this really intended or is it some kind of oversight from time where new simulink model format was created? All other matlab formats are supported but not SLX.

Cannot attach this file because:

      File format is unsupported. Use any of these formats: .bmp, .csv, .fig, .gif, .jpg, .jpeg, .m, .mlx, .mat, .mdl, .pdf, .png, .txt, .xls, .zip