Security vulnerability: .exe or other malicious must not upload by changing their header and extension .pdf, .doc, .docx, .xls, .xlsx, .jpg, .jpeg, .png etc

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C#

.NET4.5

MVC5

We have allowed to upload .pdf, .doc, .docx, .xls, .xlsx and images like .jpg, .jpeg, .png files in ASP.net MVC 5 everything working fine but my security team raised security vulnerability. Security team changing the malicious file extension as allowed extensions and changing header with supporting file type header by tool before server hit and file get uploaded on server.

We are finding the solution in asp.net MVC C# but didn't get solution to stop uploading malicious file on server through my asp.net MVC C# application.

What I have tried:

We have used following code and also used winista mime detect but unable to get proper solution. They are not returning proper mime type

public class urlmonMimeDetect
    {
        [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
        private extern static System.UInt32 FindMimeFromData(
            System.UInt32 pBC,
            [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
            [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
            System.UInt32 cbSize,
            [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
            System.UInt32 dwMimeFlags,
            out System.UInt32 ppwzMimeOut,
            System.UInt32 dwReserverd
        );

        public static string GetMimeFromFile(Stream fs)
        {

            byte[] buffer = new byte[256];
            fs.Read(buffer, 0, 256);

            try
            {
                System.UInt32 mimetype;
                FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);
                System.IntPtr mimeTypePtr = new IntPtr(mimetype);
                string mime = Marshal.PtrToStringUni(mimeTypePtr);
                Marshal.FreeCoTaskMem(mimeTypePtr);
                return mime;
            }
            catch (Exception e)
            {
                return "unknown/unknown";
            }
        }
    }