SimScale support a lot of CAD formats like IGES and STEP, but STEP/STP format is most compatible and recommended. if you are in Architectural Engineering and Construction industry (AEC) you can use STL format since its widely used in AEC industry. and also you can import directly from native CAD format like Onshape, Autodesk and Rhino.
In addition to what @Anware said it is also possible to upload assemblies in compressed archives (*.zip, *.tar.gz). Find a list of the supported file extensions below:
*.sldprt SolidWorks part
*.sldasm SolidWorks assembly (in compressed archive)
*.ipt Autodesk Inventor part
*.iam Autodesk Inventor assembly (in compressed archive)
*.3dm Rhino part
If you need any help with your project make sure to create a dedicated post in the project spotlight section. Hope that we could help!