Program.cs

using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;
using System.Text.RegularExpressions;
using QRCoder;

const int Port = 8080;
const long MaxUploadBytes = 100L * 1024 * 1024;
const string AppFolderName = "PhotoDrop";

var outputDirectory = Path.Combine(
    Environment.GetFolderPath(Environment.SpecialFolder.MyPictures),
    AppFolderName);

Directory.CreateDirectory(outputDirectory);

var builder = WebApplication.CreateBuilder(args);

builder.WebHost.UseUrls($"http://0.0.0.0:{Port}");
builder.WebHost.ConfigureKestrel(options =>
    options.Limits.MaxRequestBodySize = MaxUploadBytes);

var app = builder.Build();

app.UseDefaultFiles();
app.UseStaticFiles();

app.MapGet("/api/status", () =>
{
    var primaryAddress = GetPrimaryAddress();

    return Results.Ok(new
    {
        primaryAddress,
        addresses = GetLocalAddresses().Select(ip => $"http://{ip}:{Port}"),
        outputDirectory,
        maxUploadMb = MaxUploadBytes / 1024 / 1024
    });
});

app.MapGet("/api/qr", () =>
{
    var primaryAddress = GetPrimaryAddress();

    if (primaryAddress is null)
        return Results.NotFound();

    using var generator = new QRCodeGenerator();
    using var data = generator.CreateQrCode(primaryAddress, QRCodeGenerator.ECCLevel.Q);
    var png = new PngByteQRCode(data).GetGraphic(20);

    return Results.File(png, "image/png");
});

app.MapPost("/api/upload", async (HttpRequest request) =>
{
    if (!request.HasFormContentType)
        return Results.BadRequest(new { error = "Expected a file upload." });

    var form = await request.ReadFormAsync();
    var file = form.Files.GetFile("photo");

    if (file is null || file.Length == 0)
        return Results.BadRequest(new { error = "No file selected." });

    if (file.Length > MaxUploadBytes)
        return Results.BadRequest(new { error = "File is too large." });

    if (!file.ContentType.Equals("image/jpeg", StringComparison.OrdinalIgnoreCase))
        return Results.BadRequest(new { error = "Only JPEG images are accepted." });

    var baseName = Regex.Replace(
        Path.GetFileNameWithoutExtension(file.FileName),
        "[^a-zA-Z0-9а-яА-ЯёЁ _-]",
        "_");

    if (string.IsNullOrWhiteSpace(baseName))
        baseName = "photo";

    var path = CreateUniquePath(outputDirectory, baseName, ".jpg");

    await using var destination = File.Create(path);
    await file.CopyToAsync(destination);

    Console.WriteLine($"Received: {Path.GetFileName(path)} ({file.Length / 1024:N0} KB)");

    return Results.Ok(new
    {
        savedAs = Path.GetFileName(path)
    });
});

Console.WriteLine("PhotoDrop is running.");
Console.WriteLine($"Photos will be saved to: {outputDirectory}");

foreach (var address in GetLocalAddresses())
{
    Console.WriteLine($"Open this address on your iPhone: http://{address}:{Port}");
}

Console.WriteLine("Press Ctrl+C to stop the server.");

app.Run();

static IEnumerable<IPAddress> GetLocalAddresses() =>
    NetworkInterface.GetAllNetworkInterfaces()
        .Where(network =>
            network.OperationalStatus == OperationalStatus.Up &&
            network.NetworkInterfaceType != NetworkInterfaceType.Loopback &&
            network.GetIPProperties().GatewayAddresses.Any(gateway =>
                gateway.Address.AddressFamily == AddressFamily.InterNetwork &&
                !IPAddress.Any.Equals(gateway.Address)))
        .SelectMany(network => network.GetIPProperties().UnicastAddresses)
        .Select(unicast => unicast.Address)
        .Where(address =>
            address.AddressFamily == AddressFamily.InterNetwork &&
            !IPAddress.IsLoopback(address) &&
            IsPrivateLanAddress(address))
        .Distinct()
        .OrderBy(GetLanPriority);

static string? GetPrimaryAddress()
{
    foreach (var address in GetLocalAddresses())
        return $"http://{address}:{Port}";

    return null;
}

static bool IsPrivateLanAddress(IPAddress address)
{
    var bytes = address.GetAddressBytes();

    return bytes[0] == 192 && bytes[1] == 168 ||
           bytes[0] == 10 ||
           bytes[0] == 172 && bytes[1] is >= 16 and <= 31;
}

static int GetLanPriority(IPAddress address)
{
    var bytes = address.GetAddressBytes();

    return bytes[0] == 192 && bytes[1] == 168 ? 0 :
           bytes[0] == 10 ? 1 : 2;
}

static string CreateUniquePath(string directory, string baseName, string extension)
{
    var stamp = DateTime.Now.ToString("yyyyMMdd-HHmmss");

    for (var index = 0;; index++)
    {
        var suffix = index == 0 ? string.Empty : $"-{index}";
        var path = Path.Combine(directory, $"{baseName}-{stamp}{suffix}{extension}");

        if (!File.Exists(path))
            return path;
    }
}