.NET 從附檔名取得 ContentType (MIME Type)

前言

在 .NET Framework 4.5(+)要取得 File 的 MIME type 可以透過 MimeMapping.GetMimeMapping(String) Method
那在 .NET Core 要用什麼方法呢?

.NET Core 解法

在 .NET Core 可以使用 FileExtensionContentTypeProvider，如下

在 Services 註冊為 FileExtensionContentTypeProvider 為 Singleton

builder.Services.AddSingleton<FileExtensionContentTypeProvider>();

在API Controller 建構子中注入，

[Route("api/[controller]")]
[ApiController]
public class FilesController : ControllerBase
{
    private readonly FileExtensionContentTypeProvider _fileExtensionContentTypeProvider;
    public FilesController(FileExtensionContentTypeProvider fileExtensionContentTypeProvider)
    {
        _fileExtensionContentTypeProvider = fileExtensionContentTypeProvider
            ?? throw new System.ArgumentNullException(
                nameof(fileExtensionContentTypeProvider));
    }

    [HttpGet("{fileId}")]
    public ActionResult GetFile(string fileId)
    {
        const string DefaultContentType = "application/octet-stream";
        var pathToFile = "rmfer.pdf";

        if (!System.IO.File.Exists(pathToFile))
        {
            return NotFound();
        }

        if(!_fileExtensionContentTypeProvider.TryGetContentType(
            pathToFile, out var contentType))
        {
            contentType = DefaultContentType;
        }

        var bytes = System.IO.File.ReadAllBytes(pathToFile);
        return File(bytes, contentType, Path.GetFileName(pathToFile));
    }
}

參考資源

File extensions and MIME Types in .NET
Getting MIME Types from File Extensions in .Net Core