How should I detect the MIME type of an uploaded file in ASP.NET?

Question

How do people usually detect the MIME type of an uploaded file using ASP.NET?

Answer 1

in the aspx page:

<asp:FileUpload ID="FileUpload1" runat="server" />

in the codebehind (c#):

string contentType = FileUpload1.PostedFile.ContentType

Answer 2

The above code will not give correct content type if file is renamed and uploaded.

Please use this code for that

using System.Runtime.InteropServices;

[DllImport("urlmon.dll", CharSet = CharSet.Unicode, ExactSpelling = true, SetLastError = false)]
static extern int FindMimeFromData(IntPtr pBC,
    [MarshalAs(UnmanagedType.LPWStr)] string pwzUrl,
    [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.I1, SizeParamIndex = 3)] byte[] pBuffer,
    int cbSize,
    [MarshalAs(UnmanagedType.LPWStr)] string pwzMimeProposed,
    int dwMimeFlags, out IntPtr ppwzMimeOut, int dwReserved);

public static string getMimeFromFile(HttpPostedFile file)
{
    IntPtr mimeout;

    int MaxContent = (int)file.ContentLength;
    if (MaxContent > 4096) MaxContent = 4096;

    byte[] buf = new byte[MaxContent];
    file.InputStream.Read(buf, 0, MaxContent);
    int result = FindMimeFromData(IntPtr.Zero, file.FileName, buf, MaxContent, null, 0, out mimeout, 0);

    if (result != 0)
    {
        Marshal.FreeCoTaskMem(mimeout);
        return "";
    }

    string mime = Marshal.PtrToStringUni(mimeout);
    Marshal.FreeCoTaskMem(mimeout);

    return mime.ToLower();
}

Answer 3

While aneesh is correct in saying that the content type of the HTTP request may not be correct, I don't think that the marshalling for the unmanaged call is worth it. If you need to fall back to extension-to-mimetype mappings, just "borrow" the code from System.Web.MimeMapping.cctor (use Reflector). This dictionary approach is more than sufficient and doesn't require the native call.

Answer 4

Get MIME type from a file in ASP.NET Core

public string GetMimeType(string filePath)
{
    var provider = new FileExtensionContentTypeProvider();

    if (!provider.TryGetContentType(filePath, out var contentType))
        contentType = "application/octet-stream"; // fallback: unknown binary type
        
    return contentType;
}