C# File Upload

If you build on ASP.NET Core, a Worker Service, a Blazor server app, or .NET MAUI, you already have the only HTTP client you need: HttpClient. Uploading a file to files.link is three awaited calls, no AWSSDK.S3 NuGet package, no credential provider chain, no IAM policy JSON. That keeps your published assembly lean — the AWS SDK alone adds several megabytes and a config surface most apps never touch.

The pattern is the same three steps you would use from any language, expressed idiomatically with IHttpClientFactory and StreamContent so large files never sit fully in managed memory. Step one POSTs metadata to /v1/files/{folderId} with the raw API key in the Authorization header (no Bearer prefix) and reads the presigned url and id out of the urls array. Step two PUTs the file stream to that URL. Step three POSTs the id list to /v1/files/confirm-upload.

record Slot(string url, string id);
record UploadResponse(bool success, Slot[] urls);

public async Task<string> Upload(HttpClient http, string folderId, string path)
{
    var key = Environment.GetEnvironmentVariable("FILESLINK_KEY");
    var meta = new { filesMetadata = new[] { new { name = Path.GetFileName(path), size = new FileInfo(path).Length, type = "image/png" } } };

    // Authorization is scoped per-request so it is NEVER sent to the presigned URL.
    var create = new HttpRequestMessage(HttpMethod.Post, $"https://api.files.link/v1/files/{folderId}") { Content = JsonContent.Create(meta) };
    create.Headers.Add("Authorization", key);
    var r1 = await http.SendAsync(create);
    var slot = (await r1.Content.ReadFromJsonAsync<UploadResponse>())!.urls[0];

    await using var fs = File.OpenRead(path);
    var put = new StreamContent(fs);
    put.Headers.ContentType = new MediaTypeHeaderValue("image/png"); // must match the type sent in create
    await http.PutAsync(slot.url, put); // presigned URL — no Authorization header

    var confirm = new HttpRequestMessage(HttpMethod.Post, "https://api.files.link/v1/files/confirm-upload") { Content = JsonContent.Create(new { ids = new[] { slot.id } }) };
    confirm.Headers.Add("Authorization", key);
    await http.SendAsync(confirm);
    return slot.id;
}

Register the client with services.AddHttpClient() so connection pooling and DNS refresh are handled for you, and keep the API key in user-secrets during development and Azure Key Vault or environment variables in production. Because files.link returns a CDN-backed URL per file, you can hand that URL straight to a Blazor component, an MVC view, or a MAUI Image control.

Visibility is per file: public files resolve directly over the global CDN, private files are issued short-lived signed URLs. Storage is encrypted at rest and metered against prepaid credits — no metered egress invoice waiting at month end.