CSV or TSV — export from Excel as CSV first. Several files at once is fine.
Auto-detect
People / contacts
Companies / accounts
Detected from the column headers — a file with a name, email or LinkedIn column is treated as people.
Columns are read from your header row, so no reshaping is needed. Recognised:
name (or first + last), email, linkedin, company/account,
website/domain. Contacts match strongest-key-first: LinkedIn handle → email → name + company.
Read-only — nothing is written to Salesforce.