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

In moodle we can't make a API call to check if a file exists and we have
to rely on the URL where file will be downloaded.

When a problem occurs we get a 200 response with  a JSON document
with the reason of the error.

We don't want to always download the document as it will actually
download the full PDF file for success cases, the most common case.

Until now we been relying on the headers of a HEAD HTTP request,
interpreting JSON responses as the file being missing.

We have found at least one school that doesn't include the content-type
header on the response so he are switch the approach to inspect the
first bytes of the response and check if we are getting a PDF back from
Moodle.

lms/services/moodle.py

@@ -137,14 +137,15 @@ 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}")
+        response = self._http.request(
+            "GET", f"{file_id}&token={self.token}", headers={"Range": "bytes=0-5"}
+        )
         # 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")
+        # We don't want to download the full file so we'll do a GET request for the first bytes
+        # and check if the files is indeed a PDF.
+        # Most error response end up being JSON but not all of them are, some are HTML.
+        LOG.info("Content from Moodle file check %s", response.text)
+        return "%PDF" in response.text
 
     def page(self, course_id, page_id) -> dict | None:
         url = self._api_url(Function.GET_PAGES)

tests/unit/lms/services/moodle_test.py

@@ -150,18 +150,20 @@ def test_list_files(self, svc, http_service, contents):
         ]
 
     @pytest.mark.parametrize(
-        "header,expected",
+        "response,expected",
         [
-            ("application/json", False),
-            ("application/pdf", True),
+            ("some other content", False),
+            ("%PDF-14", True),
         ],
     )
-    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, response, expected):
+        http_service.request.return_value = Mock(text=response)
 
         assert svc.file_exists("URL") == expected
 
-        http_service.request.assert_called_once_with("HEAD", "URL&token=sentinel.token")
+        http_service.request.assert_called_once_with(
+            "GET", "URL&token=sentinel.token", headers={"Range": "bytes=0-5"}
+        )
 
     def test_list_pages(self, svc, http_service, contents):
         http_service.post.return_value.json.return_value = contents