The image editor primarily expects PNG. If it only has a DIBV5 bitmap, and it's less than or equal to 24 bits per pixel (which I assume indexed color is), it justs asks GDI+ to decode the bitmap, which is a Windows component. GDI+ is pretty crap so it wouldn't surprise me if it doesn't understand this. MSPaint has been dragged kicking and screaming from the Windows 3.1 days when this kind of image was more common, so I guess they have their own bitmap decoder. It's also possible to copy data to the clipboard in multiple formats, so if something like GIMP copies both PNG and bitmap, C2 will paste the PNG, which works. I guess Photoshop only copies an 8-bit indexed bitmap, which Windows itself doesn't understand
I don't see this being resolved, is arguably partly Photoshop's and Windows' fault, and it seems to have easy workarounds, so I think I'll just close this...