The files with .numbers extension are classified as spreadsheet file type, that’s why they are similar to the .xlsx files; but the Numbers files are created by using Apple iWork Numbers spreadsheet software.
Learn more about this file format here
GroupDocs.Conversion (in GroupDocs.Conversion.dll) Version: 18.104.22.168 (21.5)
public static readonly SpreadsheetFileType Numbers
Public Shared ReadOnly Numbers As SpreadsheetFileType
static initonly SpreadsheetFileType^ Numbers
static val Numbers: SpreadsheetFileType