```

feat(js): 简化图片下载逻辑并移除文件类型检测功能

- 移除 getImageExtension 函数和相关的内容类型映射逻辑
- 修改 buildImageFilename 函数，不再依赖内容类型参数，默认使用 .png 扩展名
- 将图片下载方式从 fetch + blob 改为 iframe 方式
- 添加 iframe 自动清理机制，默认延迟 60000ms 后移除
- 移除 Blob URL 创建和手动清理逻辑
- 优化下载成功提示信息
```

static/js/main.js

@@ -105,23 +105,9 @@ function switchMode(mode) {
     updateCostPreview(); // 切换模式时同步计费预览
 }
 
-function getImageExtension(contentType) {
-    const type = (contentType || '').split(';')[0].trim().toLowerCase();
-    const extensionMap = {
-        'image/jpeg': 'jpg',
-        'image/jpg': 'jpg',
-        'image/png': 'png',
-        'image/webp': 'webp',
-        'image/gif': 'gif',
-        'image/bmp': 'bmp',
-        'image/avif': 'avif'
-    };
-    return extensionMap[type] || 'png';
-}
-
-function buildImageFilename(url, contentType = '') {
+function buildImageFilename(url) {
     downloadFilenameSerial += 1;
-    const fallbackName = `ai-vision-image-${Date.now()}-${downloadFilenameSerial}.${getImageExtension(contentType)}`;
+    const fallbackName = `ai-vision-image-${Date.now()}-${downloadFilenameSerial}.png`;
 
     try {
         const parsed = new URL(url, window.location.href);
@@ -157,35 +143,22 @@ async function downloadImage(url, options = {}) {
     if (!url) return false;
 
     const silent = options.silent === true;
+    const cleanupDelay = options.cleanupDelay || 60000;
 
     try {
         if (!silent) showToast('正在准备下载...', 'info');
 
-        const filenameHint = buildImageFilename(url);
-        const response = await fetch(buildDownloadProxyUrl(url, filenameHint), {
-            credentials: 'same-origin'
-        });
+        const filename = buildImageFilename(url);
+        const iframe = document.createElement('iframe');
+        iframe.style.display = 'none';
+        iframe.src = buildDownloadProxyUrl(url, filename);
+        document.body.appendChild(iframe);
 
-        if (!response.ok) {
-            let message = `HTTP ${response.status}`;
-            const responseType = response.headers.get('Content-Type') || '';
-            if (responseType.includes('application/json')) {
-                const data = await response.json();
-                message = data.error || message;
-            }
-            throw new Error(message);
-        }
+        setTimeout(() => {
+            if (iframe.parentNode) iframe.parentNode.removeChild(iframe);
+        }, cleanupDelay);
 
-        const blob = await response.blob();
-        const filename = buildImageFilename(url, blob.type);
-        const blobUrl = window.URL.createObjectURL(blob);
-        const a = document.createElement('a');
-        a.href = blobUrl;
-        a.download = filename;
-        document.body.appendChild(a);
-        a.click();
-        document.body.removeChild(a);
-        setTimeout(() => window.URL.revokeObjectURL(blobUrl), 1000);
+        if (!silent) showToast('下载已开始，请留意浏览器下载列表', 'success');
         return true;
     } catch (e) {
         console.error('下载失败:', e);