OBJ is a simple Wavefront 3D geometry format. It is widely supported by editors and 3D printing workflows, with materials commonly stored in a sidecar MTL file.
Below are online converters for OBJ: convert OBJ files to other formats or create OBJ from compatible source files.
This converter shows the accepted source formats and the result format.
A damaged file or a non-standard codec can still fail even when the extension is supported.