I am trying to replace the above mention method with:
var file = DotNetNuke.Services.FileSystem.FileManager.Instance.GetFile(PortalId, ViewState["DownloadFilePath"].ToString());
DotNetNuke.Services.FileSystem.FileManager.Instance.WriteFileToResponse(file, ContentDisposition.Inline);

Was using: FileSystemUtils.DownloadFile(ViewState["DownloadFilePath"].ToString()); 

and it was working fine. 
Now I am getting :
Script control '' is not a registered script control. Script controls must be registered using RegisterScriptControl() before calling RegisterScriptDescriptors().
Parameter name: scriptControl

 

Anyone know what I am doing wrong?

that exception occurs when you are working with an ajax based control i.e. within an update panel. As this supports partial page updates it does not support fileuploads and can have problems with file downloads. You may need to disable this at the page (under page settings) or more likely within the module (see http://www.datasprings.com/resources/articles-information/ajax-w-dotnetnuke for an example)

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