if it is a csv file , then use Connect >Text file option. Try dragging your excel or csv into Tableau.
Basically the same as Deepak Rai, just make sure that under Connect that you select Text File when trying to open the file. Alternatively you can also right click on the CSV --> Open With and select Tableau.
If the problem still persists, could you provide a screenshot or details on what appears when you click "Show Details"