1. The FileName property is the Client end path while you need to use the Server side path for reading Excel file.
2. OLEDB can only read for file path. If you want to read directly from FileUpload, you need to use Closed XML.
Refer my article
Just remove the following lines
string filePath = Server.MapPath("~/Files/") + Path.GetFileName(FileUpload1.PostedFile.FileName);
FileUpload1.SaveAs(filePath);
and replace following line
using (XLWorkbook workBook = new XLWorkbook(filePath))
with
using (XLWorkbook workBook = new XLWorkbook(FileUpload1.PostedFile.InputStream))