Uses the permitted file types and compares the byte array sequence of the uploaded file against them

MimeTypeAllowed.cs

public FileType MimeTypeAllowed(byte[] file, string fileName, List<FileType> permittedMimeTypes)
{
    FileType uploadedFileType = null;

    string extension = Path.GetExtension(fileName) != null
                            ? Path.GetExtension(fileName).ToUpper()
                            : string.Empty;

    var matchedByByte = new List<FileType>();
    var matchedByByteAndExtension = new List<FileType>();

    foreach (var permittedMimeType in permittedMimeTypes)
    {
        int byteCount = permittedMimeType.ByteArray.Count();

        if (file.Take(byteCount).SequenceEqual(permittedMimeType.ByteArray))
        {
            if (!string.IsNullOrEmpty(permittedMimeType.FileExtension))
            {
                if (permittedMimeType.FileExtension.Equals(extension, StringComparison.InvariantCultureIgnoreCase))
                {
                    matchedByByteAndExtension.Add(new FileType(permittedMimeType.MimeType, file, extension));
                }
            }
            else
            {
                matchedByByte.Add(new FileType(permittedMimeType.MimeType, file, extension));
            }
        }
    }

    uploadedFileType = matchedByByteAndExtension.Any()
        ? matchedByByteAndExtension.FirstOrDefault()
        : matchedByByte.Any() ? matchedByByte.FirstOrDefault() : null;

    return uploadedFileType;
}