As it is currently the 3D viewer is rather limited, only covering GLB and GLTF files. This means that there is a lot of untapped potential not just in terms file support but in terms of audience. By supporting STL files you open yourself up as an incredibly versatile 3D printing file management software, something the space has lacked since its inception.
Additionally, Draco (GitHub - google/draco: Draco is a library for compressing and decompressing 3D geometric meshes and point clouds. It is intended to improve the storage and transmission of 3D graphics.) is an incredibly versatile compression library that is compatible with most 3D formats (including GLTF). If nothing else, this should be a high consideration to cover the aforementioned STL and a still wide audience.
Ideally, implementation would be handled via Three.js (GitHub - mrdoob/three.js: JavaScript 3D Library.), which covers the above formats and many more such as OBJ, FBX, and PLY.