This topic lists the requirements for importing stereolithography (*.stl) files.
An ASCII .stl file must start with the lower case keyword solid and end with endsolid.
solid ... facet normal 0.00 0.00 1.00 outer loop vertex 2.00 2.00 0.00 vertex -1.00 1.00 0.00 vertex 0.00 -1.00 0.00 endloop endfacet ... endsolid