EPPlus can load a file into memory. You're just not doing it that way. I think if you do this, you're less likely to run into trouble reading it from the file system. You can turn uploaded files into a byte array without having it as a file first, but in my example I'm opening an existing file. If you provide the code for how you're uploading the file, I can update my example.
byte[] file = File.ReadAllBytes(@"C:\file.xlsx");
using (MemoryStream ms = new MemoryStream(file))
using (ExcelPackage package = new ExcelPackage(ms))
{
if (package.Workbook.Worksheets.Count == 0)
strError = "Your Excel file does not contain any work sheets";
else
{
foreach (ExcelWorksheet worksheet in package.Workbook.Worksheets)
{