diff --git a/src/helpers/normalizeFiletype.ts b/src/helpers/normalizeFiletype.ts
index cf9389a..dcdce29 100644
--- a/src/helpers/normalizeFiletype.ts
+++ b/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;
   }