Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/IOFactory.php
private static function getReaderTypeFromExtension($filename)
    {
        $pathinfo = pathinfo($filename);
        if (!isset($pathinfo['extension'])) {
            return null;
        }

        switch (strtolower($pathinfo['extension'])) {
            case 'xlsx': // Excel (OfficeOpenXML) Spreadsheet
            case 'xlsm': // Excel (OfficeOpenXML) Macro Spreadsheet (macros will be discarded)
            case 'xltx': // Excel (OfficeOpenXML) Template
            case 'xltm': // Excel (OfficeOpenXML) Macro Template (macros will be discarded)
                return 'Xlsx';
            case 'xls': // Excel (BIFF) Spreadsheet
            case 'xlt': // Excel (BIFF) Template
                return 'Xls';
            case 'ods': // Open/Libre Offic Calc
            case 'ots': // Open/Libre Offic Calc Template
                return 'Ods';
            case 'slk':
                return 'Slk';
            case 'xml': // Excel 2003 SpreadSheetML
                return 'Xml';
            case 'gnumeric':
                return 'Gnumeric';
            case 'htm':
            case 'html':
                return 'Html';
            case 'csv':
                // Do nothing
                // We must not try to use CSV reader since it loads
                // all files including Excel files etc.
                return null;
            default:
                return null;
        }
    }
private static function getReaderTypeFromExtension($filename)
    {
        $pathinfo = pathinfo($filename);
        if (!isset($pathinfo['extension'])) {
            return null;
        }

        switch (strtolower($pathinfo['extension'])) {
            case 'xlsx': // Excel (OfficeOpenXML) Spreadsheet
            case 'xlsm': // Excel (OfficeOpenXML) Macro Spreadsheet (macros will be discarded)
            case 'xltx': // Excel (OfficeOpenXML) Template
            case 'xltm': // Excel (OfficeOpenXML) Macro Template (macros will be discarded)
                return 'Xlsx';
            case 'xls': // Excel (BIFF) Spreadsheet
            case 'xlt': // Excel (BIFF) Template
                return 'Xls';
            case 'ods': // Open/Libre Offic Calc
            case 'ots': // Open/Libre Offic Calc Template
                return 'Ods';
            case 'slk':
                return 'Slk';
            case 'xml': // Excel 2003 SpreadSheetML
                return 'Xml';
            case 'gnumeric':
                return 'Gnumeric';
            case 'htm':
            case 'html':
                return 'Html';
            case 'csv':
                // Do nothing
                // We must not try to use CSV reader since it loads
                // all files including Excel files etc.
                return null;
            default:
                return null;
        }
    }
Guess a reader type from the file extension, if any.

@param string $filename

@return null|string