diff --git a/RentalCar/server/controllers/documentController.js b/RentalCar/server/controllers/documentController.js
index 5690e03..43f653c 100644
--- a/RentalCar/server/controllers/documentController.js
+++ b/RentalCar/server/controllers/documentController.js
@@ -1,6 +1,14 @@
 import imagekit from "../configs/imageKit.js";
 import Document from "../models/Document.js";
 import User from "../models/User.js";
+import { getExtensionFromMime } from "../middleware/multer.js";
+
+const ALLOWED_DOCUMENT_TYPES = [
+  "DRIVING_LICENSE",
+  "ID_CARD",
+  "PASSPORT",
+  "OTHER",
+];
 
 export const uploadDocument = async (req, res) => {
   try {
@@ -8,17 +16,38 @@ export const uploadDocument = async (req, res) => {
     const { documentType } = req.body;
 
     if (!documentType) {
-      return res.status(400).json({ success: false, message: "documentType is required" });
+      return res.status(400).json({
+        success: false,
+        message: "documentType is required",
+      });
+    }
+
+    if (!ALLOWED_DOCUMENT_TYPES.includes(documentType)) {
+      return res.status(400).json({
+        success: false,
+        message: "Invalid documentType",
+      });
     }
 
     if (!req.file) {
-      return res.status(400).json({ success: false, message: "PDF file is required" });
+      return res.status(400).json({
+        success: false,
+        message: "PDF file is required",
+      });
+    }
+
+    if (!imagekit) {
+      return res.status(503).json({
+        success: false,
+        message: "Upload service is currently unavailable",
+      });
     }
 
-    // upload PDF na ImageKit
+    const extension = getExtensionFromMime(req.file.mimetype) || ".pdf";
+
     const uploadRes = await imagekit.upload({
       file: req.file.buffer.toString("base64"),
-      fileName: `doc_${documentType}_${Date.now()}.pdf`,
+      fileName: `doc_${userId}_${documentType}_${Date.now()}${extension}`,
       folder: "/documents",
     });
 
@@ -28,10 +57,20 @@ export const uploadDocument = async (req, res) => {
       fileUrl: uploadRes.url,
     });
 
-    await User.findByIdAndUpdate(userId, { $push: { documents: doc._id } });
+    await User.findByIdAndUpdate(userId, {
+      $addToSet: { documents: doc._id },
+    });
 
-    return res.json({ success: true, message: "Document uploaded", document: doc });
+    return res.status(201).json({
+      success: true,
+      message: "Document uploaded successfully",
+      document: doc,
+    });
   } catch (error) {
-    return res.status(500).json({ success: false, message: error.message });
+    console.error("uploadDocument error:", error);
+    return res.status(500).json({
+      success: false,
+      message: "Failed to upload document",
+    });
   }
-};
+};
\ No newline at end of file
diff --git a/RentalCar/server/controllers/ownerController.js b/RentalCar/server/controllers/ownerController.js
index d2b7e03..b6c14f0 100644
--- a/RentalCar/server/controllers/ownerController.js
+++ b/RentalCar/server/controllers/ownerController.js
@@ -4,6 +4,8 @@ import Car from "../models/Car.js";
 import User from "../models/User.js";
 import mongoose from "mongoose";
 
+import { getExtensionFromMime } from "../middleware/multer.js";
+
 // API to change role of user
 export const changeRoleToOwner = async (req, res) => {
   try {
@@ -45,11 +47,20 @@ export const addCar = async (req, res) => {
     }
 
     // upload na ImageKit
-    const uploadRes = await imagekit.upload({
-      file: req.file.buffer.toString("base64"),
-      fileName: `car_${Date.now()}.png`,
-      folder: "/cars",
-    });
+        if (!imagekit) {
+          return res.status(503).json({
+            success: false,
+            message: "Upload service is currently unavailable",
+          });
+        }
+
+        const extension = getExtensionFromMime(req.file.mimetype) || ".jpg";
+
+        const uploadRes = await imagekit.upload({
+          file: req.file.buffer.toString("base64"),
+          fileName: `car_${_id}_${Date.now()}${extension}`,
+          folder: "/cars",
+        });
 
     // optimizovana slika
     const optimizedImageUrl = imagekit.url({
@@ -210,11 +221,20 @@ export const updateUserImage = async (req, res) => {
       });
     }
 
-    const uploadRes = await imagekit.upload({
-      file: req.file.buffer.toString("base64"),
-      fileName: `user_${_id}_${Date.now()}.png`,
-      folder: "/users",
-    });
+        if (!imagekit) {
+          return res.status(503).json({
+            success: false,
+            message: "Upload service is currently unavailable",
+          });
+        }
+
+        const extension = getExtensionFromMime(req.file.mimetype) || ".jpg";
+
+        const uploadRes = await imagekit.upload({
+          file: req.file.buffer.toString("base64"),
+          fileName: `user_${_id}_${Date.now()}${extension}`,
+          folder: "/users",
+        });
 
     const optimizedImageUrl = imagekit.url({
       src: uploadRes.url,
diff --git a/RentalCar/server/middleware/multer.js b/RentalCar/server/middleware/multer.js
index b590e0c..d003b1f 100644
--- a/RentalCar/server/middleware/multer.js
+++ b/RentalCar/server/middleware/multer.js
@@ -1,10 +1,164 @@
 import multer from "multer";
+import path from "path";
+import imagekit from "../configs/imageKit.js";
 
 const storage = multer.memoryStorage();
 
-const upload = multer({
-  storage,
-  limits: { fileSize: 5 * 1024 * 1024 }, // 5MB
+const ALLOWED_DOCUMENT_MIME_TYPES = ["application/pdf"];
+const ALLOWED_DOCUMENT_EXTENSIONS = [".pdf"];
+
+const ALLOWED_IMAGE_MIME_TYPES = [
+  "image/jpeg",
+  "image/png",
+  "image/webp",
+];
+const ALLOWED_IMAGE_EXTENSIONS = [".jpg", ".jpeg", ".png", ".webp"];
+
+const MAX_DOCUMENT_SIZE = 5 * 1024 * 1024; // 5MB
+const MAX_IMAGE_SIZE = 5 * 1024 * 1024; // 5MB
+
+const createFileFilter = ({
+  allowedMimeTypes,
+  allowedExtensions,
+  fileLabel,
+}) => {
+  return (req, file, cb) => {
+    const mimeType = (file.mimetype || "").toLowerCase();
+    const extension = path.extname(file.originalname || "").toLowerCase();
+
+    const isMimeAllowed = allowedMimeTypes.includes(mimeType);
+    const isExtensionAllowed = allowedExtensions.includes(extension);
+
+    if (!isMimeAllowed || !isExtensionAllowed) {
+      const error = new Error(
+        `Invalid ${fileLabel} type. Allowed: ${allowedExtensions.join(", ")}`
+      );
+      error.statusCode = 400;
+      error.code = "INVALID_FILE_TYPE";
+      return cb(error);
+    }
+
+    cb(null, true);
+  };
+};
+
+const createMemoryUpload = ({ maxFileSize, allowedMimeTypes, allowedExtensions, fileLabel }) =>
+  multer({
+    storage,
+    limits: { fileSize: maxFileSize },
+    fileFilter: createFileFilter({
+      allowedMimeTypes,
+      allowedExtensions,
+      fileLabel,
+    }),
+  });
+
+export const documentUpload = createMemoryUpload({
+  maxFileSize: MAX_DOCUMENT_SIZE,
+  allowedMimeTypes: ALLOWED_DOCUMENT_MIME_TYPES,
+  allowedExtensions: ALLOWED_DOCUMENT_EXTENSIONS,
+  fileLabel: "document",
 });
 
-export default upload;
+export const imageUpload = createMemoryUpload({
+  maxFileSize: MAX_IMAGE_SIZE,
+  allowedMimeTypes: ALLOWED_IMAGE_MIME_TYPES,
+  allowedExtensions: ALLOWED_IMAGE_EXTENSIONS,
+  fileLabel: "image",
+});
+
+export const applyUpload = (multerMiddleware) => {
+  return (req, res, next) => {
+    multerMiddleware(req, res, (err) => {
+      if (!err) return next();
+
+      if (err instanceof multer.MulterError) {
+        if (err.code === "LIMIT_FILE_SIZE") {
+          return res.status(413).json({
+            success: false,
+            message: "File too large. Maximum allowed size is 5MB.",
+          });
+        }
+
+        return res.status(400).json({
+          success: false,
+          message: err.message || "Upload error",
+        });
+      }
+
+      const statusCode = err.statusCode || 400;
+      return res.status(statusCode).json({
+        success: false,
+        message: err.message || "Invalid upload",
+      });
+    });
+  };
+};
+
+export const ensureFilePresent = (fieldName = "file") => {
+  return (req, res, next) => {
+    if (!req.file) {
+      return res.status(400).json({
+        success: false,
+        message: `Missing required file field: ${fieldName}`,
+      });
+    }
+    next();
+  };
+};
+
+export const ensureImageKitConfigured = (req, res, next) => {
+  if (!imagekit) {
+    return res.status(503).json({
+      success: false,
+      message: "Upload service is currently unavailable",
+    });
+  }
+  next();
+};
+
+export const getExtensionFromMime = (mimeType = "") => {
+  const normalized = mimeType.toLowerCase();
+
+  switch (normalized) {
+    case "application/pdf":
+      return ".pdf";
+    case "image/jpeg":
+      return ".jpg";
+    case "image/png":
+      return ".png";
+    case "image/webp":
+      return ".webp";
+    default:
+      return "";
+  }
+};
+
+export const isPdfMagicNumberValid = (buffer) => {
+  if (!buffer || buffer.length < 4) return false;
+
+  return (
+    buffer[0] === 0x25 && // %
+    buffer[1] === 0x50 && // P
+    buffer[2] === 0x44 && // D
+    buffer[3] === 0x46    // F
+  );
+};
+
+export const ensureValidPdfSignature = (req, res, next) => {
+  if (!req.file || !req.file.buffer) {
+    return res.status(400).json({
+      success: false,
+      message: "PDF file is required",
+    });
+  }
+
+  if (!isPdfMagicNumberValid(req.file.buffer)) {
+    return res.status(400).json({
+      success: false,
+      message: "Invalid PDF file signature",
+    });
+  }
+
+  next();
+};
\ No newline at end of file
diff --git a/RentalCar/server/routes/documentRoutes.js b/RentalCar/server/routes/documentRoutes.js
index 37ed09b..e9bae9d 100644
--- a/RentalCar/server/routes/documentRoutes.js
+++ b/RentalCar/server/routes/documentRoutes.js
@@ -1,11 +1,15 @@
 import express from "express";
-import multer from "multer";
 import { protect } from "../middleware/auth.js";
 import { uploadDocument } from "../controllers/documentController.js";
-
+import {
+  applyUpload,
+  documentUpload,
+  ensureFilePresent,
+  ensureImageKitConfigured,
+  ensureValidPdfSignature,
+} from "../middleware/multer.js";
 
 const router = express.Router();
-const upload = multer({ storage: multer.memoryStorage() });
 
 /**
  * @openapi
@@ -19,7 +23,7 @@ const upload = multer({ storage: multer.memoryStorage() });
  * /api/document/upload:
  *   post:
  *     tags: [Document]
- *     summary: Upload dokumenta (PDF/JPG/PNG) - multipart
+ *     summary: Upload dokumenta (samo PDF) - multipart
  *     security:
  *       - bearerAuth: []
  *     requestBody:
@@ -28,21 +32,33 @@ const upload = multer({ storage: multer.memoryStorage() });
  *         multipart/form-data:
  *           schema:
  *             type: object
+ *             required: [file, documentType]
  *             properties:
  *               file:
  *                 type: string
  *                 format: binary
- *               type:
+ *                 description: PDF dokument, maksimalno 5MB
+ *               documentType:
  *                 type: string
- *                 example: "DRIVERS_LICENSE"
+ *                 enum: [DRIVING_LICENSE, ID_CARD, PASSPORT, OTHER]
  *     responses:
  *       200:
  *         description: Dokument uploadovan
- *         content:
- *           application/json:
- *             schema:
- *               $ref: '#/components/schemas/DocumentUploadResponse'
+ *       400:
+ *         description: Neispravan dokument ili neispravan request
+ *       413:
+ *         description: Fajl je prevelik
+ *       503:
+ *         description: Upload servis nije dostupan
  */
-router.post("/upload", protect, upload.single("file"), uploadDocument);
+router.post(
+  "/upload",
+  protect,
+  ensureImageKitConfigured,
+  applyUpload(documentUpload.single("file")),
+  ensureFilePresent("file"),
+  ensureValidPdfSignature,
+  uploadDocument
+);
 
-export default router;
+export default router;
\ No newline at end of file
diff --git a/RentalCar/server/routes/ownerRoutes.js b/RentalCar/server/routes/ownerRoutes.js
index 15e021f..19ebaa8 100644
--- a/RentalCar/server/routes/ownerRoutes.js
+++ b/RentalCar/server/routes/ownerRoutes.js
@@ -1,6 +1,11 @@
 import express from "express";
 import { protect } from "../middleware/auth.js";
-import upload from "../middleware/multer.js";
+import {
+  applyUpload,
+  imageUpload,
+  ensureFilePresent,
+  ensureImageKitConfigured,
+} from "../middleware/multer.js";
 import { requireRole } from "../middleware/roles.js";
 
 import {
@@ -79,7 +84,9 @@ ownerRouter.post(
   "/add-car",
   protect,
   requireRole("owner", "admin"),
-  upload.single("image"),
+  ensureImageKitConfigured,
+  applyUpload(imageUpload.single("image")),
+  ensureFilePresent("image"),
   addCar
 );
 
@@ -251,7 +258,9 @@ ownerRouter.post(
   "/update-image",
   protect,
   requireRole("owner", "admin"),
-  upload.single("image"),
+  ensureImageKitConfigured,
+  applyUpload(imageUpload.single("image")),
+  ensureFilePresent("image"),
   updateUserImage
 );
 export default ownerRouter;