Excel is only displaying as text from the preview pane.
Currently, this is considered as expected behavior. The Excel preview is designed to display as text only. It will not show macros, formatting, borders, etc.
Engineering team has already considered this to be an enhancement but for future release (no commitment yet).
CUB-33565 (SAP internal)
Excel Preview pane, preview pane excel, Jam Excel Preview , KBA , LOD-SF-JAM-ADM , Administrator Request , Problem