File import

The first step of an import. Drop, choose or paste a CSV, TSV or Excel file and see what was read: the rows, the columns and a preview. The separator, encoding, header row and each column's kind are detected, and anything that looks off (a guessed encoding, garbled characters, rows that split unevenly, an unclosed quote) is named with the one change that fixes it. Reads XLSX in the browser with no spreadsheet library.