How to check mime type/ internal mime type

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C#

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C#3.5

C#5.0

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There is a case in file upload,I have a image file and I am uploading that one is fine, and in other case have change the extension of .exe to png /jpeg , then how can we validate that file in this case.

What I have tried:

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);

       if (mime == "application/pdf")
       {
           // upload the File because file is valid // and checking other file type
           Response.Write("This is Valid File");
       }
       else
       {
           //  file is Invalid
           Response.Write("This is Invalid File");

       }