# How to Implement File Uploads in Topcoat: Complete Multipart Handling Guide

> Learn how to implement file uploads in Topcoat with this comprehensive guide. Discover Topcoat's Multipart extractor for seamless multipart form data handling.

- Repository: [Tokio/topcoat](https://github.com/tokio-rs/topcoat)
- Tags: how-to-guide
- Published: 2026-07-31

---

**Topcoat provides a first-class `Multipart` extractor in the router crate that handles `multipart/form-data` requests by streaming fields through the multer library and enforcing configured body limits.**

The tokio-rs/topcoat framework simplifies HTTP file upload handling through its integrated multipart support. To implement file uploads in Topcoat, you use the `Multipart` type which automatically parses request bodies and provides streaming access to uploaded fields and form data. This extractor lives in [`crates/topcoat-router/src/content/multipart.rs`](https://github.com/tokio-rs/topcoat/blob/main/crates/topcoat-router/src/content/multipart.rs) and integrates directly with the router's request processing pipeline.

## Understanding the Multipart Extractor

### Core Implementation and Dependencies

According to the Topcoat source code, the `Multipart` extractor implements `FromRequest` to automatically handle incoming requests. When a route handler accepts a `Multipart` parameter, Topcoat validates the `Content-Type` header, extracts the multipart boundary string, and delegates parsing to the **multer** library. The extractor respects the router's `BodyLimit` configuration, returning a **413 Content Too Large** response when uploads exceed the configured limit.

### Field Access Methods

Each field yielded by `next_field()` exposes metadata through methods defined in the `Field` struct:

- `name()` returns the form field name from the `Content-Disposition` header
- `file_name()` provides the original filename for file uploads
- `content_type()` reveals the MIME type of the field
- `bytes()` and `text()` load the entire payload into memory as `Bytes` or `String`
- `chunk()` and the `Stream` implementation allow incremental processing for large files

## Processing File Uploads in Route Handlers

To handle uploads, define a route using the `#[route(POST "...")]` attribute and accept `mut multipart: Multipart` as a handler parameter. Iterate through fields using `while let Some(field) = multipart.next_field().await?` to process each part individually.

```rust
use topcoat::{
    Result,
    router::{content::multipart::Multipart, route},
    Cx,
};

#[route(POST "/api/upload")]
async fn upload(mut multipart: Multipart) -> Result<&'static str> {
    // Process each part of the multipart request
    while let Some(field) = multipart.next_field().await? {
        // Identify the field by its name attribute
        let name = field.name().unwrap_or("<unknown>");

        // If the field has a filename, treat it as a file upload
        if let Some(filename) = field.file_name() {
            // Read the whole file into memory (or stream with `chunk()`)
            let data = field.bytes().await?;

            // Here you could write `data` to disk, S3, etc.
            println!("Uploaded file `{filename}` ({:?} bytes) in field `{name}`", data.len());
        } else {
            // Non‑file field – treat as plain text
            let text = field.text().await?;
            println!("Form field `{name}`: {text}");
        }
    }

    Ok("upload processed")
}

```

This pattern, demonstrated in [`examples/request-response/src/main.rs`](https://github.com/tokio-rs/topcoat/blob/main/examples/request-response/src/main.rs), distinguishes between file fields (which have a filename) and regular form fields, processing each appropriately while providing access to metadata like original filenames and content types.

## Configuring Upload Size Limits

By default, the extractor reads through the router's body limit layer, which restricts maximum payload sizes. For upload routes requiring larger limits, apply the `BodyLimit` layer to specific paths:

```rust
use topcoat::router::BodyLimit;

router
    .route(upload)
    .layer(BodyLimit::max(64 * 1024 * 1024).at("/api/upload"));

```

This configuration permits uploads up to 64 MB on the `/api/upload` endpoint while maintaining stricter default limits on other routes. The `.at()` method ensures the limit applies only to the specified path.

## Error Handling and HTTP Status Codes

The `Multipart` extractor produces specific HTTP responses for error conditions encountered during parsing:

- **400 Bad Request** (`invalid_boundary`) when the `Content-Type` header lacks `multipart/form-data` or contains a malformed boundary
- **413 Content Too Large** when the payload exceeds the configured `BodyLimit` before or during streaming
- **400 Bad Request** for malformed multipart data encountered while reading fields through the multer library

## Summary

- The `Multipart` extractor in [`crates/topcoat-router/src/content/multipart.rs`](https://github.com/tokio-rs/topcoat/blob/main/crates/topcoat-router/src/content/multipart.rs) provides native support for `multipart/form-data` uploads without external middleware
- Access uploaded files through `next_field()` and inspect metadata using `name()`, `file_name()`, and `content_type()`
- Control memory usage by choosing between `bytes()` (full load) and `chunk()` (streaming) when reading field data
- Configure route-specific size limits using the `BodyLimit` layer to prevent **413 Content Too Large** errors on upload endpoints
- The extractor relies on the multer library for standards-compliant parsing while respecting Topcoat's body limit constraints

## Frequently Asked Questions

### What happens if the request Content-Type is not multipart/form-data?

The extractor returns a **400 Bad Request** with the `invalid_boundary` error code. This occurs immediately during request processing because the `FromRequest` implementation validates the `Content-Type` header before attempting to parse the body.

### How do I stream large files instead of loading them into memory?

Use the `chunk()` method or treat the `Field` as a `Stream` implementation instead of calling `bytes()`. This allows you to process data incrementally as it arrives from the client, avoiding memory exhaustion when handling large uploads.

### Can I configure different size limits for different routes?

Yes. Apply the `BodyLimit` layer to specific routes using the `.at()` method as implemented in [`crates/topcoat-router/src/content/multipart.rs`](https://github.com/tokio-rs/topcoat/blob/main/crates/topcoat-router/src/content/multipart.rs). This allows you to set higher limits for upload endpoints while maintaining stricter default restrictions on other routes.

### What library does Topcoat use internally for parsing multipart data?

Topcoat delegates multipart parsing to the **multer** library, which handles boundary extraction, header parsing, and streaming field data while respecting the router's body limit constraints enforced through `limited(cx, body)`.