diff --git a/apps/api/app/services/demo/shared_document_service.py b/apps/api/app/services/demo/shared_document_service.py
index eb6699c82..97f6b4a04 100644
--- a/apps/api/app/services/demo/shared_document_service.py
+++ b/apps/api/app/services/demo/shared_document_service.py
@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 import asyncio
-import mimetypes
+from pathlib import PurePath
 from typing import Any
 from urllib.parse import quote
 
@@ -99,8 +99,9 @@ async def get_catalog(self, db: AsyncSession) -> dict[str, Any]:
                 continue
             metadata: dict[str, Any] = revision.document_metadata or {}
             mimeType: str = (
-                mimetypes.guess_type(document.source_file_name or document.title)[0]
-                or "application/octet-stream"
+                JobFileStorage.get_content_type(
+                    PurePath(document.source_file_name or document.title).suffix
+                )
             )
             source: dict[str, Any] = {
                 "demo_source_id": document.demo_source_id,
@@ -206,7 +207,7 @@ async def get_media_url(
             )
         if not key:
             raise NotFoundException(resource="Demo original", resource_id=source_id)
-        mimeType: str = mimetypes.guess_type(fileName)[0] or "application/octet-stream"
+        mimeType: str = JobFileStorage.get_content_type(PurePath(fileName).suffix)
         # Redirects let the storage service handle Range without proxying large
         # original files through an API worker. The GET signature overrides MIME.
         return await asyncio.to_thread(
diff --git a/apps/api/tests/contract/test_shared_demo_corpus_contract.py b/apps/api/tests/contract/test_shared_demo_corpus_contract.py
index ef783f84e..3c4ce43ba 100644
--- a/apps/api/tests/contract/test_shared_demo_corpus_contract.py
+++ b/apps/api/tests/contract/test_shared_demo_corpus_contract.py
@@ -464,16 +464,43 @@ async def test_demo_agent_tools_pin_history_and_private_isolation(
 
 
 @pytest.mark.asyncio
+@pytest.mark.parametrize(
+    ("file_name", "original_mime"),
+    [
+        ("original.pdf", "application/pdf"),
+        (
+            "original.docx",
+            "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+        ),
+        (
+            "original.DOCX",
+            "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+        ),
+    ],
+)
 async def test_demo_revision_media_validation_and_job_retention(
-    developer_api_client_factory: ClientFactory, monkeypatch: pytest.MonkeyPatch
+    developer_api_client_factory: ClientFactory,
+    monkeypatch: pytest.MonkeyPatch,
+    file_name: str,
+    original_mime: str,
 ) -> None:
     async with developer_api_client_factory() as client:
         job = await create_demo(
-            client, source_id="media-demo", file_name="original.pdf"
+            client, source_id="media-demo", file_name=file_name
         )
         revision = await publish_demo(job, has_page=True)
+        import mimetypes
+        from pathlib import PurePath
         from shared.services.storage.job_file_storage import JobFileStorage
 
+        # Slim runtime images need not include the operating system MIME registry.
+        mimetypes.init()
+        monkeypatch.delitem(mimetypes.types_map, PurePath(file_name).suffix.lower(), raising=False)
+        catalog = (await client.get("/api/v1/demo/catalog")).json()
+        source = next(item for item in catalog["sources"] if item["demo_source_id"] == "media-demo")
+        assert source["mime_type"] == original_mime
+        assert source["original_file"]["mime_type"] == original_mime
+
         calls: list[dict[str, Any]] = []
 
         def sign(key: str, **arguments: Any) -> str:
@@ -487,7 +514,7 @@ class SignedStorage:
             JobFileStorage, "storage_adapter", property(lambda self: SignedStorage())
         )
         for path, mime in (
-            ("original", "application/pdf"),
+            ("original", original_mime),
             ("assets/images/rocket.png", "image/png"),
             ("assets/tables/launch.html", "text/html"),
         ):