The XLTM file extension represents files that are generated by Microsoft Excel as Macro-enabled template files. XLTM files are similar to XLTX in structure other than that the later doesn't support creating template files with macros.
Learn more about this file format here
GroupDocs.Conversion (in GroupDocs.Conversion.dll) Version: 18.104.22.168 (21.3.1)
public static readonly SpreadsheetFileType Xltm
Public Shared ReadOnly Xltm As SpreadsheetFileType
static initonly SpreadsheetFileType^ Xltm
static val Xltm: SpreadsheetFileType