Microsoft Excel Macro-Enabled Template (.xltm) 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.Viewer (in GroupDocs.Viewer.dll) Version: 20.6
public static readonly FileType XLTM
Public Shared ReadOnly XLTM As FileType
static initonly FileType^ XLTM
static val XLTM: FileType