Add test for flattening select_one_from_file

(test currently failing)

tests/test_utils_flatten_content.py

@@ -106,6 +106,17 @@ def test_flatten_select_or_other():
     assert row0['type'] == 'select_one xyz or_other'
     assert 'select_from_list_name' not in row0
 
+def test_flatten_select_one_from_file():
+    s1 = {
+        'survey': [
+            {'type': 'select_one_from_file', 'file': 'fruits.csv'}
+        ]
+    }
+    flatten_content(s1, in_place=True)
+    row0 = s1['survey'][0]
+    assert row0['type'] == 'select_one_from_file fruits.csv'
+    assert 'file' not in row0
+
 
 def test_flatten_select():
     s1 = {'survey': [{'type': 'select_one', 'select_from_list_name': 'aaa'}]}