finish api and docs validation fixes

server/index.ts

@@ -198,7 +198,7 @@ function uploadedFileToPart(file: Express.Multer.File): GeminiPart {
   return {
     inlineData: {
       data: file.buffer.toString('base64'),
-      mimeType: file.mimetype || 'application/octet-stream',
+      mimeType: inferMimeType(file),
     },
   };
 }
@@ -219,6 +219,30 @@ function normalizeTextInput(value: unknown) {
   return value == null ? '' : String(value);
 }
 
+function inferMimeType(file: Express.Multer.File) {
+  if (file.mimetype && file.mimetype !== 'application/octet-stream') {
+    return file.mimetype;
+  }
+
+  const ext = path.extname(file.originalname || '').toLowerCase();
+  const mimeByExt: Record<string, string> = {
+    '.png': 'image/png',
+    '.jpg': 'image/jpeg',
+    '.jpeg': 'image/jpeg',
+    '.webp': 'image/webp',
+    '.gif': 'image/gif',
+    '.pdf': 'application/pdf',
+    '.txt': 'text/plain',
+    '.md': 'text/plain',
+    '.csv': 'text/csv',
+    '.json': 'application/json',
+    '.html': 'text/html',
+    '.htm': 'text/html',
+  };
+
+  return mimeByExt[ext] || 'application/octet-stream';
+}
+
 async function buildParts(body: GenerateRequest, files: Express.Multer.File[] = []) {
   const prompt = normalizeTextInput(body.prompt || body.instruction).trim();
   if (!prompt) {