PrompterOne: Skill for Codex

Implementing File Uploads in ASP.NET Core Minimal APIs

When to Use

• File upload endpoints in ASP.NET Core minimal APIs (.NET 8+)
• Handling IFormFile or IFormFileCollection parameters
• When you need size limits, content type validation, or streaming large files

When Not to Use

• MVC controllers → [FromForm] IFormFile works directly with attributes
• Simple JSON body → no file upload needed
• Very large files (> 1GB) → use streaming with MultipartReader instead

Inputs

Input	Required	Description
File parameter(s)	Yes	IFormFile or IFormFileCollection
Size limits	Yes	Max file/request size
Allowed types	No	Content type or extension restrictions

Workflow

Step 1: CRITICAL — Understand IFormFile Binding in Minimal APIs

// In .NET 8+ minimal APIs, IFormFile binds automatically from multipart/form-data
// when it is the only complex parameter.
app.MapPost("/upload", (IFormFile file) => ...);

// CRITICAL: When you mix files with other form fields, use [FromForm] on all
// form-bound parameters (or group them into a single [FromForm] DTO).
app.MapPost("/upload-with-metadata",
    ([FromForm] IFormFile file, [FromForm] string description) =>
{
    return Results.Ok(new { file.FileName, Description = description });
});

// Multiple files: IFormFileCollection also binds automatically from multipart/form-data.
// You only need [FromForm] if you mix it with other form fields, as shown above.
app.MapPost("/upload-multiple", (IFormFileCollection files) =>
{
    return Results.Ok(files.Select(f => new { f.FileName, f.Length }));
});

Step 2: CRITICAL — File Size Limits Are Separate from Request Size Limits

// CRITICAL: There are TWO different size limits and you need to configure BOTH

// 1. Request body size limit (Kestrel level) — default is 30MB
builder.WebHost.ConfigureKestrel(options =>
{
    options.Limits.MaxRequestBodySize = 10 * 1024 * 1024; // 10 MB
});

// 2. Form options — multipart body length limit — default is 128MB
builder.Services.Configure<FormOptions>(options =>
{
    options.MultipartBodyLengthLimit = 10 * 1024 * 1024; // 10 MB
    options.ValueLengthLimit = 1024 * 1024; // 1 MB for form values
    options.MultipartHeadersLengthLimit = 16384; // 16 KB for section headers
});

// COMMON MISTAKE: Only increasing Kestrel MaxRequestBodySize
// upload still fails because FormOptions.MultipartBodyLengthLimit is exceeded

// COMMON MISTAKE: Only increasing FormOptions
// upload fails with "Request body too large" from Kestrel before reaching form parsing

// CRITICAL: Per-endpoint override with RequestSizeLimit attribute
app.MapPost("/upload-large", [RequestSizeLimit(200_000_000)] (IFormFile file) =>
{
    return Results.Ok(new { file.FileName, file.Length });
});

// CRITICAL: To disable the limit entirely (for streaming):
app.MapPost("/upload-unlimited", [DisableRequestSizeLimit] async (HttpContext context) =>
{
    // Handle manually
});