diff --git a/packages/scaffold/src/utils/mime.ts b/packages/scaffold/src/utils/mime.ts
index 396bb00..ce54f11 100644
--- a/packages/scaffold/src/utils/mime.ts
+++ b/packages/scaffold/src/utils/mime.ts
@@ -1,16 +1,16 @@
 import { extname } from "node:path";
 
-const mimeTypes: { [key: string]: string[] } = {
-  "application/json": ["json"],
-  "application/x-xpinstall": ["xpi"],
+const mimeTypes: { [key: string]: string } = {
+  json: "application/json",
+  xpi: "application/x-xpinstall",
 };
 
 export function getMimeTypeByFileName(filename: string) {
   const ext = extname(filename);
 
   for (const type in mimeTypes) {
-    if (mimeTypes[type].includes(ext))
-      return type;
+    if (ext === `.${type}`)
+      return mimeTypes[type];
   }
 
   return undefined;
diff --git a/packages/scaffold/test/unit/mime.test.ts b/packages/scaffold/test/unit/mime.test.ts
new file mode 100644
index 0000000..6ca91f5
--- /dev/null
+++ b/packages/scaffold/test/unit/mime.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+import { getMimeTypeByFileName } from "../../src/utils/mime.js";
+
+describe("mime", () => {
+  describe("getMimeTypeByFileName", () => {
+    it("should return application/json for json files", () => {
+      expect(getMimeTypeByFileName("test.json")).toEqual("application/json");
+    });
+
+    it("should return application/x-xpinstall for xpi files", () => {
+      expect(getMimeTypeByFileName("test.xpi")).toEqual("application/x-xpinstall");
+    });
+
+    it("should return undefined for unknown file types", () => {
+      expect(getMimeTypeByFileName("test.unknown")).toBeUndefined();
+    });
+  });
+});