In urlmon.dll,there’s a function called FindMimeFromData.
Read the first(up to) 256 bytes from the file and pass it to FindMimeFromData function.
Sample code:
using System.Runtime.InteropServices;    

[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 string getMimeFromFile(string filename)    
if (!File.Exists(filename))    
throw new FileNotFoundException(filename + " not found");    
byte[] buffer = new byte[256];    
using (FileStream fs = new FileStream(filename, FileMode.Open))    
if (fs.Length >= 256)    
                                fs.Read(buffer, 0, 256);    
                                fs.Read(buffer, 0, (int)fs.Length);    
System.UInt32 mimetype;    
FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);    
System.IntPtr mimeTypePtr = new IntPtr(mimetype);    
string mime = Marshal.PtrToStringUni(mimeTypePtr);    
return mime;    
catch (Exception e)    
return "unknown/unknown";