diff --git a/src/lib/file.ts b/src/lib/file.ts
index 07cfa2cd..cb681bde 100644
--- a/src/lib/file.ts
+++ b/src/lib/file.ts
@@ -8,7 +8,7 @@ const mime = new Map([
 
 export function ext(mimeType: string): string {
     if (mime.has(mimeType)) {
-        return mime.get(mimeType)
+        return mime.get(mimeType)!
     }
 
     return mimeType