Response.AddHeader: I have to save the file, can't open it right there

I 'm using code below to descrypt a file and after it's decrypted, I want to open it. I click on the link to open the file and I get 3 options: save file, open file, download file...

If I click, "open", the file opens but I get a msg that the content cant be open/corrupted.
If I click "save", then go to the folder and open the file..the file is ok and opens just fine.

I dont know why that is. Dont think it's the encryption/decryption that's doing it.
I see the file's name as "whatever.pdf"...so the extension is there. This is the code I have:

Byte[] bytesFile = null;
        string fileExt = string.Empty;
        string fileName = string.Empty;

        using (SqlDataReader sdr = objSqlCmd.ExecuteReader())
        {
            while (sdr.Read())
            {
                bytesFile = (Byte[])sdr[2];
                 fileExt = sdr[1].ToString();
                 fileName = sdr[0].ToString();

            }
        }

        var decryptFile = bytesFile; //GetEncryptedFile(docId.Text);
        var decrypted = EncryptData(decryptFile, key, key, false);
      

        Response.Buffer = true;
        Response.Charset = "";
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        // Response.ContentType = sdr[1].ToString();

        Response.AddHeader("content-disposition", "attachment;filename=" + fileName);
        Response.ContentType = fileExt;
        Response.BinaryWrite(decrypted);
        Response.Flush();
        Response.End();

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The decryption code:

 private static byte[] EncryptData(byte[] input, byte[] desKey, byte[] desIV, bool encrytp = true)
      {
          MemoryStream fin = new MemoryStream(input);
          MemoryStream fout = new MemoryStream();
          int rd = 1024;
          byte[] bin = new byte[rd];
          long rdlen = 0;
          long totlen = fin.Length;
          int len;

          DES des = new DESCryptoServiceProvider();
          CryptoStream encStream = null;
          if (encrytp)
          {
              encStream = new CryptoStream(fout, des.CreateEncryptor(desKey, desIV), CryptoStreamMode.Write);
          }
          else
          {
              encStream = new CryptoStream(fout, des.CreateDecryptor(desKey, desIV), CryptoStreamMode.Write);
          }
          while (rdlen < totlen)
          {
              len = fin.Read(bin, 0, rd);
              encStream.Write(bin, 0, len);
              rdlen = rdlen + len;
          }

          encStream.Close();
          fout.Close();
          fin.Close();

         
          return fout.ToArray();
      }

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