FileActionResult.cs

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.StaticFiles;
using Microsoft.Net.Http.Headers;
using System.Threading.Tasks;

namespace ActionResults;

public class FileActionResult : IActionResult
{
    private readonly byte[] _contentFile;
    private readonly string _contentType;
    private readonly string _filename;

    public FileActionResult(byte[] contentFile, string filename)
    {
        _contentFile = contentFile;
        _filename = filename;
        _contentType = GetContentType(filename);
    }

    public async Task ExecuteResultAsync(ActionContext context)
    {
        context.HttpContext.Response.Headers["x-file-name"] = _filename;
        context.HttpContext.Response.Headers[HeaderNames.ContentType] = _contentType;
        context.HttpContext.Response.Headers[HeaderNames.ContentLength] = _contentFile.Length.ToString(provider: default);
        context.HttpContext.Response.Headers[HeaderNames.ContentDisposition] = $"{System.Net.Mime.DispositionTypeNames.Inline}; filename=\"{_filename}\";";

        await context.HttpContext.Response.Body.WriteAsync(_contentFile, 0, _contentFile.Length).ConfigureAwait(false);
    }

    private string GetContentType(string fileName)
    {
        var provider = new FileExtensionContentTypeProvider();
        if (!provider.TryGetContentType(fileName, out string contentType))
            contentType = "application/octet-stream";

        return contentType;
    }
}