Download file using Response.TransmitFile is giving checksum mismatch

I am using Response.TransmitFile to enable download feature on my web site and the file size is 2GB. It was working fine till last month. But now customers are complaining that the file downloaded from the size is corrupt. We tried on multiple browser and found the checksum is not matching. When we manually copy the files, checksum is matching. Files at source are not corrupt and is verified. Below is the code snippet used. Could this checksum mismatch is induced by web server? The file downloaded ends up being slightly more in file size compared to file in source. Please help.

            UserInfo objUserInfo = (UserInfo)Session["USER_INFO"];
            string strDocname = ((LinkButton)sender).CommandArgument.ToString();
            strDocname = strDocname.Replace("\r\n", "");
            strSource = ConfigurationManager.AppSettings["DOWNLOAD_PATH"].ToString() + Path.GetFileName(strDocname);
            new Log().logInfo("GotoSystem_onCommand", strSource);
            FileInfo file = new FileInfo(strDocname);
            Response.ContentType = "application/octet-stream";
            Response.AppendHeader("Content-Disposition", "attachment; filename=" + strDocname + "");
            UpdateDownloadLog(objUserInfo.GetUserAcctId(), strDocname, Session["BRAND"].ToString());
Ashwini RaoSystem ArchitectAsked:
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Brijesh GandhiProgrammerCommented:
Add Below Header in Code and try it
Response.AddHeader("Content-Length", file.Length.ToString());

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And also make sure this I think it should be
   FileInfo file = new FileInfo(strSource );

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Ashwini RaoSystem ArchitectAuthor Commented:

Thanks so much. It worked, but not sure why we need to add this line. Could you explain this for my understanding?
Brijesh GandhiProgrammerCommented:
In past I also got this kind of error . So I am aware of this .. For more detail you can refer below link
Content Length
Need of Content Length
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