diff --git a/src/converters/assimp.ts b/src/converters/assimp.ts
index 69f3258..42e1b6f 100644
--- a/src/converters/assimp.ts
+++ b/src/converters/assimp.ts
@@ -121,20 +121,24 @@ export async function convert(
   execFile: ExecFileFn = execFileOriginal, // to make it mockable
 ): Promise<string> {
   return new Promise((resolve, reject) => {
-    execFile("assimp", ["export", filePath, targetPath], (error, stdout, stderr) => {
-      if (error) {
-        reject(`error: ${error}`);
-      }
+    execFile(
+      "assimp",
+      ["export", filePath, targetPath, `-f${convertTo}`],
+      (error, stdout, stderr) => {
+        if (error) {
+          reject(`error: ${error}`);
+        }
 
-      if (stdout) {
-        console.log(`stdout: ${stdout}`);
-      }
+        if (stdout) {
+          console.log(`stdout: ${stdout}`);
+        }
 
-      if (stderr) {
-        console.error(`stderr: ${stderr}`);
-      }
+        if (stderr) {
+          console.error(`stderr: ${stderr}`);
+        }
 
-      resolve("Done");
-    });
+        resolve("Done");
+      },
+    );
   });
 }
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;
   }