How to avoid malicious file upload while uploading via file upload in ASP.NET with C#?

See more:

C#

ASP.NET

I want to avoid malicious file upload while uploading via file upload in asp.net with c#. If i change the text file to pdf extension and upload ,I am able to validate using mime type for the PDF file(application/pdf) but not able to do so for excel file(xls/xlsx) with same code (mime type coming as application/octet-stream). I am using URL moniker dll and asp.net with c#. I am uploading only excel and PDF files and validating the file extensions initially.

What I have tried:

Code is as follows:

private extern static System.UInt32 FindMimeFromData(
      System.UInt32 pBC,
      [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
      [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
      System.UInt32 cbSize,
      [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
      System.UInt32 dwMimeFlags,
      out System.UInt32 ppwzMimeOut,
      System.UInt32 dwReserverd
  );
HttpPostedFile file = FileUpload1.PostedFile;
        byte[] document = new byte[file.ContentLength];
        file.InputStream.Read(document, 0, file.ContentLength);
        System.UInt32 mimetype;
        FindMimeFromData(0, null, document, 256, null, 0, out mimetype, 0);
        System.IntPtr mimeTypePtr = new IntPtr(mimetype);
        string mime = Marshal.PtrToStringUni(mimeTypePtr);
        Marshal.FreeCoTaskMem(mimeTypePtr);