Handle non-string image content types

tests/image-utils.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { isValidImageMimeType, bufferToBase64 } from '../src/utils/image.js';
+import { isValidImageMimeType, bufferToBase64, normalizeContentType } from '../src/utils/image.js';
 
 describe('isValidImageMimeType', () => {
   it('accepts valid image MIME types', () => {
@@ -72,3 +72,13 @@ describe('bufferToBase64', () => {
     expect(result).toMatch(/^[A-Za-z0-9+/]+=*$/);
   });
 });
+
+describe('normalizeContentType', () => {
+  it('uses the fallback when Axios exposes a non-string header value', () => {
+    expect(normalizeContentType(['image/png'])).toBe('image/jpeg');
+  });
+
+  it('removes header parameters from a string content type', () => {
+    expect(normalizeContentType('image/webp; charset=binary')).toBe('image/webp');
+  });
+});