Base moodle's file_exist method on the file content instead of headers

lms/services/moodle.py

@@ -137,14 +137,17 @@ def list_files(self, course_id: int):
     def file_exists(self, file_id) -> bool:
         """Check if the file exists in the course."""
         # Moodle file IDs are URLs, but they need the token to be accessible
-        response = self._http.request("HEAD", f"{file_id}&token={self.token}")
-        # Moodle API doesn't use status codes, we can't rely on that.
-        # We don't want to download the full file so we'll do a HEAD request and assume:
-        #   - JSON response, it's an error response
-        #   - Anything else, it's  the file we are after
-
-        LOG.info("Headers from Moodle file check %s", response.headers)
-        return not response.headers["content-type"].startswith("application/json")
+        response = self._http.request(
+            "GET", f"{file_id}&token={self.token}", headers={"Range": "bytes=0-1024"}
+        )
+        # API doesn't use status codes, we can't rely on that.
+        if content_type := response.headers.get("content-type"):
+            # JSON response, assume it's an error response
+            return not content_type.startswith("application/json")
+
+        # If the Moodle server doesn't return the content-type header, check the first bytes of the response
+        # and check if the files is indeed a PDF.
+        return b"%PDF" in response.content
 
     def page(self, course_id, page_id) -> dict | None:
         url = self._api_url(Function.GET_PAGES)

tests/unit/lms/services/moodle_test.py

@@ -149,19 +149,24 @@ def test_list_files(self, svc, http_service, contents):
             }
         ]
 
+    @pytest.mark.parametrize("content", [b"some other content", b"%PDF-14"])
     @pytest.mark.parametrize(
-        "header,expected",
-        [
-            ("application/json", False),
-            ("application/pdf", True),
-        ],
+        "headers",
+        [{}, {"content-type": "application/json"}, {"content-type": "application/pdf"}],
     )
-    def test_file_exists(self, svc, http_service, header, expected):
-        http_service.request.return_value = Mock(headers={"content-type": header})
+    def test_file_exists(self, svc, http_service, headers, content):
+        http_service.request.return_value = Mock(content=content, headers=headers)
 
-        assert svc.file_exists("URL") == expected
+        expected = (
+            not headers.get("content-type", "").startswith("application/json")
+            if headers.get("content-type")
+            else b"%PDF" in content
+        )
 
-        http_service.request.assert_called_once_with("HEAD", "URL&token=sentinel.token")
+        assert svc.file_exists("URL") == expected
+        http_service.request.assert_called_once_with(
+            "GET", "URL&token=sentinel.token", headers={"Range": "bytes=0-1024"}
+        )
 
     def test_list_pages(self, svc, http_service, contents):
         http_service.post.return_value.json.return_value = contents