Updating logic and adding tests

I have changed the logic as suggested in the review.

For the unit tests, I have made direct function call to test the logic.

falcon/util/misc.py

@@ -380,10 +380,10 @@ def secure_filename(filename: str, max_length: Optional[int] = None) -> str:
 
     if max_length is not None and len(filename) > max_length:
         name, ext = os.path.splitext(filename)
-        if max_length < len(ext):
-            filename = ext[:max_length]
-        else:
+        if max_length - len(ext) >= 1:
             filename = name[: max_length - len(ext)] + ext
+        else:
+            filename = filename[:max_length]
 
     return filename
 

tests/test_media_multipart.py

@@ -10,6 +10,7 @@
 from falcon import testing
 from falcon.media.multipart import MultipartParseOptions
 from falcon.util import BufferedReader
+from falcon.util.misc import secure_filename
 
 try:
     import msgpack
@@ -747,6 +748,14 @@ def test_unsupported_charset(client):
     }
 
 
+def test_max_length_filename(client):
+    filename = secure_filename('averylongfilename.txt', 17)
+    assert filename == 'averylongfile.txt'
+
+    filename = secure_filename('file.withlongextension', 9)
+    assert filename == 'file.with'
+
+
 def test_filename_star(client):
     # NOTE(vytas): Generated by requests_toolbelt 0.9.1 on Py2
     #   (interestingly, one gets a "normal" filename on Py3.7).