UserItem.CSVImport.create_from_file: wrong file-extension check

[jacalata]

Users.create_from_file (in users_endpoint.py) validates the file with "csv" not in filepath before parsing. That test:

• accepts non-CSV files whose path merely contains the substring csv (e.g. report.csv.bak, mycsv.txt, /home/csvuser/data.json)
• rejects valid files whose extension is uppercase (e.g. USERS.CSV)

Neither matches the error message ("Only csv files are accepted").

Fix would be Path(filepath).suffix.lower() == ".csv".

Note: create_from_file is already deprecated (emits DeprecationWarning and is being replaced by bulk_add). Two paths:

1. Patch under the deprecation window so the last few callers get correct behavior until removal.
2. Just delete the method when the deprecation window closes and don't touch it in the meantime.

Filed as follow-up to a review comment on PR #1812 (out of scope for that PR's refactor).