diff --git a/backend/app/main.py b/backend/app/main.py
index 2727e5e..2cf85d0 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -691,7 +691,14 @@ async def scan(
             status_code=413,
             detail=f"Header indicates file is too large. Maximum upload size is {MAX_UPLOAD_MB}MB.",
         )
+    filename = project.filename or ""
 
+    if not filename.lower().endswith(".zip"):
+        raise HTTPException(
+            status_code=415,
+            detail="Only .zip archives are accepted. Please compress your project and try again.",
+        )
+    
     job_id = next(tempfile._get_candidate_names())
     job_dir = WORK_ROOT / job_id
     ensure_dir(job_dir)
diff --git a/backend/tests/test_upload_security.py b/backend/tests/test_upload_security.py
index 92725ee..81fbd70 100644
--- a/backend/tests/test_upload_security.py
+++ b/backend/tests/test_upload_security.py
@@ -44,3 +44,20 @@ def test_upload_aborts_when_stream_exceeds_limit():
 
     assert response.status_code == 413
     assert "Actual file size exceeds" in response.json()["detail"]
+def test_upload_rejects_non_zip_file():
+    """
+    Test that the server immediately rejects uploads whose filename
+    does not end with .zip.
+    """
+    files = {
+        "project": ("test.txt", io.BytesIO(b"hello"), "text/plain")
+    }
+    data = {"project": "test_project"}
+
+    response = client.post("/scan", files=files, data=data)
+
+    assert response.status_code == 415
+    assert (
+        response.json()["detail"]
+        == "Only .zip archives are accepted. Please compress your project and try again."
+    )
\ No newline at end of file