To guess MIME type based on byte[]: https://github.com/hey-red/Mime

mimeController.cs

public class ValuesController : Controller
{
    private readonly IMimeMappingService _mimeMappingService;
 
    public ValuesController(IMimeMappingService mimeMappingService)
    {
        _mimeMappingService = mimeMappingService;
    }
 
    [HttpGet]
    public string Get(string fileName)
    {
\!h        return _mimeMappingService.Map(fileName);
    }
}

mimeGuesser.cs

using HeyRed.Mime;

// (Optionally) You can set path to magic database file manually.
MimeGuesser.MagicFilePath = "/path/to/magic/file";

// Guess mime type of file(overloaded method takes byte array or stream as arg.)
MimeGuesser.GuessMimeType("path/to/file"); //=> image/jpeg

// Get extension of file(overloaded method takes byte array or stream as arg.)
MimeGuesser.GuessExtension("path/to/file"); //=> jpeg

// Get mime type and extension of file(overloaded method takes byte array or stream as arg.)
MimeGuesser.GuessFileType("path/to/file"); //=> FileType

// Advanced: Want more than just the mime type? Use the Magic class
string calc = @"C:\Windows\System32\calc.exe";
var magic = new Magic(MagicOpenFlags.MAGIC_NONE);
magic.Read(calc); //=> PE32+ executable (GUI) x86-64, for MS Windows

// Check encoding:
string textFile = @"F:\Temp\file.txt";
var magic = new Magic(MagicOpenFlags.MAGIC_MIME_ENCODING);
magic.Read(textFile); //=> Output: utf-8

mimeInStartup.cs

using Microsoft.AspNetCore.StaticFiles;


public void ConfigureServices(IServiceCollection services)
{
\!h    var provider = new FileExtensionContentTypeProvider();
\!h    provider.Mappings.Add(".dnct", "application/dotnetcoretutorials");
\!h    services.AddSingleton<IMimeMappingService>(new MimeMappingService(provider));
 
    services.AddMvc();
}

mimeService.cs

public interface IMimeMappingService
{
    string Map(string fileName);
}
 
public class MimeMappingService : IMimeMappingService
{
    private readonly FileExtensionContentTypeProvider _contentTypeProvider;
 
    public MimeMappingService(FileExtensionContentTypeProvider contentTypeProvider)
    {
        _contentTypeProvider = contentTypeProvider;
    }
 
    public string Map(string fileName)
    {
        string contentType;
        if (!_contentTypeProvider.TryGetContentType(fileName, out contentType))
        {
            contentType = "application/octet-stream";
        }
        return contentType;
    }
}