Implementing File Uploads in ASP.NET Core Minimal APIs

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

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) =>