Unless a recruiter explicitly tells you otherwise, wear business formal attire to an investment banking interview. In practice, that usually means a matching suit, a professional shirt or blouse, conservative shoes, and restrained accessories. For candidates who wear ties, a tie is normally part of the interview outfit.
Bank dress codes vary by firm, office, group, and location, and some employees dress more casually during an ordinary workday. That does not make casual clothing appropriate for an interview. Your interview outfit should show that you understand the industry’s client-facing standards, not imitate what a banker might wear on a quiet Friday.
If the invitation specifies a dress code, follow it. You can ask the recruiter for clarification if a phrase such as “business attire” seems ambiguous, but there is usually no need to contact them simply to confirm that a suit is acceptable. When no guidance is provided, the conservative option carries less risk.