In addition to what @rszoeke said, I’d like to add that we also support binary STL files. The binary format is much more size-efficient than the ASCII format. Many CAD tools allow you to choose if you want to create a binary or ASCII STL, and there are also tools which convert between both formats.
The only drawback is that binary STLs are unable to encode information about how faces are split within one solid. The model will consist of one solid with one face. You can use the geometry operation “Surface splitting” to reconstruct the information about different faces within the model.
For ASCII STLs we allow the (non-standard but widely used) method to provide several “solid” sections within one file which correspond to one face each.