Sniff image MIME for ZK viewer blobs so save-as gets a real extension.

frontend/src/stores/zkView.ts

@@ -3,6 +3,7 @@ import { ref } from 'vue';
 import { api } from '@/plugins/axios';
 import type { GetZkUploadCiphertextResult } from '@/types/upload/GetZkUploadCiphertextResult';
 import { base64ToBytes } from '@/utils/encoding/base64ToBytes';
+import { detectImageMimeType } from '@/utils/file/detectImageMimeType';
 import { toArrayBuffer } from '@/utils/toArrayBuffer';
 import { decryptPasswordProtectedZkUpload } from '@/utils/upload/decryptPasswordProtectedZkUpload';
 import { decryptZkUpload } from '@/utils/upload/decryptZkUpload';
@@ -67,7 +68,9 @@ export const useZkViewStore = defineStore('zkView', () => {
             throw new Error('Could not build image blob');
         }
 
-        imageObjectUrl.value = URL.createObjectURL(new Blob([buffer]));
+        const mimeType = await detectImageMimeType(plaintext);
+
+        imageObjectUrl.value = URL.createObjectURL(new Blob([buffer], { type: mimeType }));
     };
 
     return {