Infer Content-Type for document image endpoint (#15368)

## Summary

Fixes [#15367](https://github.com/infiniflow/ragflow/issues/15367) —
`GET /api/v1/documents/images/<image_id>` always returned `Content-Type:
image/JPEG` even for PNG/WebP chunk images and extensioned thumbnails.

## Related Issue

Fixes #15367

## Change Type

- [x] Bug fix
- [x] Regression tests
- [ ] New feature
- [ ] Refactor

## What Changed

- Added `_detect_image_content_type_from_bytes()` —
PNG/JPEG/GIF/WebP/BMP magic-byte detection
- Added `_content_type_for_document_image()` — object-key extension via
`CONTENT_TYPE_MAP`, then magic bytes, else `application/octet-stream`
- **`get_document_image()`** — set inferred `Content-Type` instead of
hardcoded `image/JPEG`
- Also guards missing storage blob (`Image not found.`) to avoid
`make_response(None)` (same handler; complements #15365)

## Files Changed

| File | Change |
|------|--------|
| `api/apps/restful_apis/document_api.py` | MIME inference helpers +
handler update |
|
`test/testcases/test_web_api/test_document_app/test_document_metadata.py`
| 3 unit tests |

## Validation

```bash
cd /root/gittensor/ragflow
pytest test/testcases/test_web_api/test_document_app/test_document_metadata.py::TestDocumentMetadataUnit::test_get_document_image_content_type_from_object_extension_unit -v
pytest test/testcases/test_web_api/test_document_app/test_document_metadata.py::TestDocumentMetadataUnit::test_get_document_image_content_type_from_magic_bytes_unit -v
pytest test/testcases/test_web_api/test_document_app/test_document_metadata.py::TestDocumentMetadataUnit::test_get_document_image_missing_blob_unit -v
```

## Test Plan

- [x] `.png` object key → `image/png`
- [x] Extensionless chunk key + PNG bytes → `image/png` (magic bytes)
- [x] Missing blob → 4xx `"Image not found."`
- [ ] CI green

test/testcases/test_web_api/test_document_app/test_document_metadata.py

@@ -450,6 +450,82 @@ class TestDocumentMetadataUnit:
         assert res["code"] == 500
         assert "download boom" in res["message"]
 
+    @pytest.mark.p2
+    def test_get_document_image_content_type_from_object_extension_unit(self, document_app_module, monkeypatch):
+        module = document_app_module
+
+        class _Headers(dict):
+            def set(self, key, value):
+                self[key] = value
+
+        class _ImageResponse:
+            def __init__(self, data):
+                self.data = data
+                self.headers = _Headers()
+
+        png_bytes = (
+            b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x01"
+            b"\x00\x00\x00\x01\x08\x06\x00\x00\x00\x1f\x15\xc4\x89"
+            b"\x00\x00\x00\nIDATx\x9cc\x00\x01\x00\x00\x05\x00\x01"
+            b"\r\n-\xb4\x00\x00\x00\x00IEND\xaeB`\x82"
+        )
+
+        async def fake_thread_pool_exec(*_args, **_kwargs):
+            return png_bytes
+
+        async def fake_make_response(data):
+            return _ImageResponse(data)
+
+        monkeypatch.setattr(module, "thread_pool_exec", fake_thread_pool_exec)
+        monkeypatch.setattr(module, "make_response", fake_make_response)
+        res = _run(module.get_document_image("kb1-object.png"))
+        assert isinstance(res, _ImageResponse)
+        assert res.headers["Content-Type"] == "image/png"
+
+    @pytest.mark.p2
+    def test_get_document_image_content_type_from_magic_bytes_unit(self, document_app_module, monkeypatch):
+        module = document_app_module
+
+        class _Headers(dict):
+            def set(self, key, value):
+                self[key] = value
+
+        class _ImageResponse:
+            def __init__(self, data):
+                self.data = data
+                self.headers = _Headers()
+
+        png_bytes = (
+            b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x01"
+            b"\x00\x00\x00\x01\x08\x06\x00\x00\x00\x1f\x15\xc4\x89"
+            b"\x00\x00\x00\nIDATx\x9cc\x00\x01\x00\x00\x05\x00\x01"
+            b"\r\n-\xb4\x00\x00\x00\x00IEND\xaeB`\x82"
+        )
+
+        async def fake_thread_pool_exec(*_args, **_kwargs):
+            return png_bytes
+
+        async def fake_make_response(data):
+            return _ImageResponse(data)
+
+        monkeypatch.setattr(module, "thread_pool_exec", fake_thread_pool_exec)
+        monkeypatch.setattr(module, "make_response", fake_make_response)
+        res = _run(module.get_document_image("kb1-a1b2c3d4e5f6"))
+        assert isinstance(res, _ImageResponse)
+        assert res.headers["Content-Type"] == "image/png"
+
+    @pytest.mark.p2
+    def test_get_document_image_missing_blob_unit(self, document_app_module, monkeypatch):
+        module = document_app_module
+
+        async def fake_thread_pool_exec(*_args, **_kwargs):
+            return None
+
+        monkeypatch.setattr(module, "thread_pool_exec", fake_thread_pool_exec)
+        res = _run(module.get_document_image("kb1-object-key"))
+        assert res["code"] == RetCode.DATA_ERROR
+        assert res["message"] == "Image not found."
+
 
     @pytest.mark.skip(reason="Moved to /api/v1/documents/images/<image_id>")
     def test_get_image_success_and_exception_unit(self, document_app_module, monkeypatch):