(scm net http multipart)

multipart/form-data parsing (RFC 7578)

Overview

(scm net http multipart) parses multipart/form-data request bodies (RFC 7578) — the format browsers use for file uploads. Each parsed part is an alist with the keys name, filename, content-type, and body; part-ref reads a field from a part.

Common uses

Find the boundary from the Content-Type header, then parse the body:

(import (scm net http multipart))

(multipart-boundary "multipart/form-data; boundary=abc123")  ;; => "abc123"

(define parts
  (parse-multipart body (multipart-boundary content-type)))

(part-ref (car parts) 'name)        ;; => "avatar"
(part-ref (car parts) 'filename)    ;; => "photo.png"

Use parse-multipart when each character of the body is one byte (text), and parse-multipart-bytes when the body is a bytevector — the latter keeps binary attachments (images, PDFs) intact.

4 bindings

multipart-boundary

Syntax: (multipart-boundary content-type)
Library: (scm net http multipart)
Description: Extracts the boundary= parameter from a Content-Type header
  value. Returns the boundary string (without surrounding quotes), or #f
  if content-type is not multipart/form-data or has no boundary.
Example:
  (multipart-boundary "multipart/form-data; boundary=abc123") => "abc123"
  (multipart-boundary "text/plain") => #f

parse-multipart

Syntax: (parse-multipart body boundary)
Library: (scm net http multipart)
Description: Parses a multipart/form-data body (passed as a byte-per-char
  string) with the given boundary string. Returns a list of part alists,
  each with keys 'name, 'filename (or #f), 'content-type (or #f), 'body.
  Use parse-multipart-bytes when handling binary uploads.
Example:
  (parse-multipart body (multipart-boundary (http-request-header req "Content-Type")))

parse-multipart-bytes

Syntax: (parse-multipart-bytes body boundary)
Library: (scm net http multipart)
Description: Like parse-multipart but operates on a bytevector body. Part
  bodies in the result are bytevectors (subcopies of the input), so binary
  uploads (images, PDFs, ...) round-trip exactly.
Example:
  (parse-multipart-bytes (http-request-body-bytes req)
                         (multipart-boundary
                           (http-request-header req "Content-Type")))

part-ref

Syntax: (part-ref part key)
Library: (scm net http multipart)
Description: Returns the value of key ('name, 'filename, 'content-type, 'body)
  in a part alist, or #f if missing.
Example:
  (part-ref p 'name) => "avatar"
  (part-ref p 'filename) => "face.png"