If you have pasted a 3D file, it is opened in 3D mode, as usual.

Files with 2D elements cannot be pasted in 3D mode.

When files have been pasted in 3D mode, only 3D files can be additionally pasted.

To be able to combine a pasted file with other files in a working window, you must convert the pasted file in 3D mode by projecting it into a 2D file (camera button). Once in 2D mode, as many additional 3D files as you wish can be pasted or 2D files placed in the working window. Naturally, you can also use the tools and functions in 2D mode as normal.