Hello,
An invalid-format message does not mean that the extension is wrong, but that the content of the file is not formatted in a way that PennController can parse. What separators do you use in your CSV file? Ideally you want to use commas, but tabs would work too. I think any other separator character (eg. simple space) will fail. Also, double-check how you handle quotes and escaping characters: if you have text cells containing the same character as your separator character (eg. typically a comma) you want to make sure that when you save your file as CSV, those cells are surrounded by quotes, to indicate that the character (eg. the comma) is not actually separating two columns in those cases.
Jeremy