You may saw before the article Create XLSX/MHTML file from internal table in background and you may have use it without any issues before, but it seems that in newer releases of SAP (definitelly >= 7.50) ...
Many people still asks how to create field catalog for CL_GUI_ALV_GRID or REUSE_ALV_GRID_DISPLAY on a base of internal table. This is very easy since we have SALV classes, you could see it in my article Create ...