[IMP] survey_extra_fields : handle file question on page navigation

* Show a stored file (after navigating back) the same way as a freshly
  selected one: a "filename + Remove file" chip, input hidden until removal.
* Keep the stored file when a page is re-submitted without a new selection
  (instead of overwriting it with a skipped answer).
* "Remove file" now sends a sentinel to actually delete the stored file,
  so the removal persists across navigation.

survey_extra_fields/tests/test_survey_file.py

@@ -94,19 +94,38 @@ class TestSurveyFileSaveLines(TestSurveyFileCommon):
         self.assertEqual(lines.value_file, new_b64.encode())
         self.assertEqual(lines.value_file_fname, "second.pdf")
 
-    def test_save_file_then_skip(self):
-        """Uploading a file then submitting empty marks line as skipped."""
+    def test_save_file_then_empty_keeps_file(self):
+        """Submitting empty after a file keeps it (file inputs cannot be
+        pre-filled when navigating back, so an empty answer must not erase it)."""
         answer = self._add_answer(self.survey, self.survey_manager.partner_id)
         file_json = json.dumps({"data": self.file_b64, "name": self.file_name})
         answer._save_lines(self.question_file, file_json)
 
         answer._save_lines(self.question_file, "")
 
+        line = answer.user_input_line_ids.filtered(
+            lambda l: l.question_id == self.question_file
+        )
+        self.assertEqual(len(line), 1)
+        self.assertFalse(line.skipped)
+        self.assertEqual(line.value_file, self.file_b64.encode())
+        self.assertEqual(line.value_file_fname, self.file_name)
+
+    def test_save_file_explicitly_cleared(self):
+        """Submitting the 'cleared' sentinel after a file removes it."""
+        answer = self._add_answer(self.survey, self.survey_manager.partner_id)
+        file_json = json.dumps({"data": self.file_b64, "name": self.file_name})
+        answer._save_lines(self.question_file, file_json)
+
+        answer._save_lines(self.question_file, json.dumps({"cleared": True}))
+
         line = answer.user_input_line_ids.filtered(
             lambda l: l.question_id == self.question_file
         )
         self.assertEqual(len(line), 1)
         self.assertTrue(line.skipped)
+        self.assertFalse(line.value_file)
+        self.assertFalse(line.value_file_fname)
 
 
 class TestSurveyFileConstraints(TestSurveyFileCommon):