It is recommended to specify the ambient temperature and world's temperature gradient, since this dictates how objects with no specified temperature appear. Here's an example that sets the ambient temperature to 300 kelvin, and the temperature gradient to 0.1 Kelvin/meter:
Copy this example world code into a text editor and save it as import_mesh.sdf. This is a simple world SDF file, which you can learn more about on the SDF website.
Scroll all the way to the bottom of the file until you see the include tag section following the <!-- mesh --> comment line.