[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/models/survey_user_input.py

@@ -31,14 +31,25 @@ class SurveyUserInput(models.Model):
             ])
             if old_answers and not overwrite_existing:
                 raise UserError(_("This answer cannot be overwritten."))
+            if not answer and any(line.value_file for line in old_answers):
+                # No new file was submitted: a file input cannot be pre-filled
+                # by the browser when navigating back to a previous page, so an
+                # empty answer here does not mean the user removed their file.
+                # Keep the previously uploaded file instead of overwriting it
+                # with a skipped answer.
+                return
             vals = {
                 "user_input_id": self.id,
                 "question_id": question.id,
                 "skipped": False,
                 "answer_type": "file",
             }
-            if answer:
-                file_data = json.loads(answer)
+            file_data = json.loads(answer) if answer else {}
+            if file_data.get("cleared"):
+                # The user explicitly removed the file: drop the stored data and
+                # mark the line as skipped.
+                vals.update(answer_type=None, skipped=True, value_file=False, value_file_fname=False)
+            elif file_data:
                 file_b64 = file_data.get("data", "")
                 file_name = file_data.get("name", "")
                 self._check_file_constraints(question, file_b64, file_name)