fix(assimp): write real file extensions for non-extension format ids

Follow-up to the previous commit. With `-f<format>` now passed to
assimp, every advertised target exports successfully — but seven of
them still produce files with assimp's internal format id as the
extension, which no third-party viewer recognises:

  glb2 → .glb2   (really binary glTF 2.0, should be .glb)
  gltf2 → .gltf2 (text glTF 2.0, should be .gltf)
  objnomtl → .objnomtl   (OBJ without .mtl, should be .obj)
  stlb → .stlb   (binary STL, should be .stl)
  plyb → .plyb   (binary PLY, should be .ply)
  fbxa → .fbxa   (ASCII FBX, should be .fbx)
  assjson → .assjson   (JSON dump, should be .json)

Map these in `normalizeOutputFiletype`, the same helper that already
handles jpeg→jpg, latex→tex, markdown→md. The format id on `convertTo`
is untouched, so `assimp -f<id>` still selects the correct
encoding/variant — only the output filename changes.

assbin, assxml, and pbrt are both the format id and the canonical
extension, so they need no mapping.

src/helpers/normalizeFiletype.ts

@@ -31,6 +31,24 @@ export const normalizeOutputFiletype = (filetype: string): string => {
     case "markdown_mmd":
     case "markdown":
       return "md";
+    // assimp format ids that aren't real file extensions — map to the
+    // canonical extension for the underlying format so the output file
+    // opens in third-party viewers. The format id is still passed to
+    // `assimp export -f<id>` so the right encoding/variant is produced.
+    case "glb2":
+      return "glb";
+    case "gltf2":
+      return "gltf";
+    case "objnomtl":
+      return "obj";
+    case "stlb":
+      return "stl";
+    case "plyb":
+      return "ply";
+    case "fbxa":
+      return "fbx";
+    case "assjson":
+      return "json";
     default:
       return lowercaseFiletype;
   }