If you're expecting people to send you files, you should require them to use the format you decided, and warn them any other format will be ignored. And make sure you process invalid formats correctly by gracefully reporting an error instead of crashing...
I suggest you redirect your users to the appropriate RFC 4180, in case they want to be fully compliant with the CSV standard.
If you're nice, you can add an option for them to enter the separator they used when they upload the file (and make sure you handle cases when they mistyped it or willfully gave the wrong one).
If you're even nicer, you can read the first line and try different commonly used separators (<space>, \t, ,, ;, ...) and choose the one that gives you the biggest number of tokens.
Whenever you're dealing with user input, you'll have to put a lot of effort (and code) validating every corner-case.
Further note: There is a standard written for CSV files: RFC 4180. I would refer the submitters to that document. FYI, the Apache Commons CSV library supports this standard format in
CSVFormat.RFC4180(as well as variations).