could not open the file.

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ASP.NET

hi
i have created an app that uploads files to a folder and then takes the files from that folder and convert it to images.
here is the code

if (FileUpload1.HasFile)
        {
          
            string file = System.IO.Path.GetFullPath(FileUpload1.PostedFile.FileName);
            string filname = FileUpload1.PostedFile.FileName;
//UPLOADS THE FILE INTO THE FOLDER
            FileUpload1.PostedFile.SaveAs(Server.MapPath("~\\NewFolder1\\") + FileUpload1.FileName);
//TAKES THE PATH and file
            string strPath = Server.MapPath(Request.ApplicationPath) + "~/NewFolder1/" + filname;
//assign the file to ppt
 ApplicationClass pptApplication = new ApplicationClass();
            Presentation pptPresentation = pptApplication.Presentations.Open(strPath, MsoTriState.msoFalse,
            MsoTriState.msoFalse, MsoTriState.msoFalse);

            var slides = new List<string>();
            targetPath = Server.MapPath("~/NewFolder1/");
            targetPath += "\\{0}";
            for (var i = 1; i <= pptPresentation.Slides.Count; i++)
            {
             .
             .
</string>

but when i run the app it gives the follwoing error.
System.Runtime.InteropServices.COMException: PowerPoint could not open the file.
Presentation pptPresentation = pptApplication.Presentations.Open(strPath, MsoTriState.msoFalse....

please tell me where is the problem and what i need to correct