Kavita/API/Services/DownloadService.cs

using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using API.Entities;
using API.Interfaces.Services;
using Microsoft.AspNetCore.StaticFiles;

namespace API.Services
{
    public interface IDownloadService
    {
        Task<(byte[], string, string)> GetFirstFileDownload(IEnumerable<MangaFile> files);
        string GetContentTypeFromFile(string filepath);
    }
    public class DownloadService : IDownloadService
    {
        private readonly IDirectoryService _directoryService;
        private readonly FileExtensionContentTypeProvider _fileTypeProvider = new FileExtensionContentTypeProvider();

        public DownloadService(IDirectoryService directoryService)
        {
            _directoryService = directoryService;
        }

        /// <summary>
        /// Downloads the first file in the file enumerable for download
        /// </summary>
        /// <param name="files"></param>
        /// <returns></returns>
        public async Task<(byte[], string, string)> GetFirstFileDownload(IEnumerable<MangaFile> files)
        {
            var firstFile = files.Select(c => c.FilePath).First();
            return (await _directoryService.ReadFileAsync(firstFile), GetContentTypeFromFile(firstFile), Path.GetFileName(firstFile));
        }

        public string GetContentTypeFromFile(string filepath)
        {
            // Figures out what the content type should be based on the file name.
            if (!_fileTypeProvider.TryGetContentType(filepath, out var contentType))
            {
                contentType = Path.GetExtension(filepath).ToLowerInvariant() switch
                {
                    ".cbz" => "application/zip",
                    ".cbr" => "application/vnd.rar",
                    ".cb7" => "application/x-compressed",
                    ".epub" => "application/epub+zip",
                    ".7z" => "application/x-7z-compressed",
                    ".7zip" => "application/x-7z-compressed",
                    ".pdf" => "application/pdf",
                    _ => contentType
                };
            }

            return contentType;
        }
    }
}