You can read more details about splitting areas in the manual.
Hover the cursor over the upper right corner of the files window until it turns into a cross. Left click and drag to "split" the window. Once you've done that, use the editor selector to switch the window to the 3D viewport.
If you have trouble splitting the window, you can hover over the edge between the file window and the outliner until the cursor turns into a two-headed error. Right click and select "Vertical Split". Drag the line to the right and then left click.
Splitting the window:
Changing the window type:
Alternative way to split