Add content_type option to Multipart for multipart/related and mixed

Multipart#initialize now accepts a content_type: keyword argument,
defaulting to "multipart/form-data". This enables multipart/related,
multipart/mixed, and other multipart content types without requiring
users to manually construct the body.

Closes #1.

Author: sferik

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- `Multipart` accepts a `content_type:` keyword to support `multipart/related`,
+  `multipart/mixed`, and other multipart content types. Defaults to
+  `multipart/form-data`.
+  ([#1](https://github.com/httprb/form_data/issues/1))
 - `FormData::File#close` for closing file handles opened from String paths or
   Pathnames. When a File is created from an existing IO, `close` is a no-op.
   ([#27](https://github.com/httprb/form_data/issues/27))

lib/http/form_data/multipart.rb

@@ -12,6 +12,9 @@ module FormData
     class Multipart
       include Readable
 
+      # Default MIME type for multipart form data
+      DEFAULT_CONTENT_TYPE = "multipart/form-data"
+
       # Returns the multipart boundary string
       #
       # @example
@@ -23,14 +26,19 @@ class Multipart
 
       # Creates a new Multipart form data instance
       #
-      # @example
+      # @example Basic form data
       #   Multipart.new({ foo: "bar" })
       #
+      # @example With custom content type
+      #   Multipart.new(parts, content_type: "multipart/related")
+      #
       # @api public
       # @param [Enumerable, Hash, #to_h] data form data key-value pairs
       # @param [String] boundary custom boundary string
-      def initialize(data, boundary: self.class.generate_boundary)
-        @boundary = boundary.to_s.freeze
+      # @param [String] content_type MIME type for the Content-Type header
+      def initialize(data, boundary: self.class.generate_boundary, content_type: DEFAULT_CONTENT_TYPE)
+        @boundary     = boundary.to_s.freeze
+        @content_type = content_type
         @io = CompositeIO.new(parts(data).flat_map { |part| [glue, part] } << tail)
       end
 
@@ -54,7 +62,7 @@ def self.generate_boundary
       # @api public
       # @return [String]
       def content_type
-        "multipart/form-data; boundary=#{@boundary}"
+        "#{@content_type}; boundary=#{@boundary}"
       end
 
       # Returns form data content size for Content-Length

sig/http/form_data/multipart.rbs

@@ -3,11 +3,15 @@ module HTTP
     class Multipart
       include Readable
 
+      DEFAULT_CONTENT_TYPE: String
+
       # Returns the multipart boundary string
       attr_reader boundary: String
 
+      @content_type: _ToS
+
       # Creates a new Multipart form data instance
-      def initialize: (untyped data, ?boundary: _ToS) -> void
+      def initialize: (untyped data, ?boundary: _ToS, ?content_type: _ToS) -> void
 
       # Generates a boundary string for multipart form data
       def self.generate_boundary: () -> String

test/http/form_data/multipart_test.rb

@@ -171,6 +171,29 @@ def test_content_type_with_user_defined_boundary
     assert_equal "multipart/form-data; boundary=my-boundary", form_data.content_type
   end
 
+  def test_content_type_with_custom_type
+    form_data = HTTP::FormData::Multipart.new(@params, boundary: "b", content_type: "multipart/related")
+
+    assert_equal "multipart/related; boundary=b", form_data.content_type
+  end
+
+  def test_content_type_with_multipart_mixed
+    form_data = HTTP::FormData::Multipart.new(@params, boundary: "b", content_type: "multipart/mixed")
+
+    assert_equal "multipart/mixed; boundary=b", form_data.content_type
+  end
+
+  def test_content_type_default_is_form_data
+    assert_equal "multipart/form-data", HTTP::FormData::Multipart::DEFAULT_CONTENT_TYPE
+  end
+
+  def test_content_type_converts_to_string
+    form_data = HTTP::FormData::Multipart.new(@params, boundary: "b", content_type: :"multipart/related")
+
+    assert_equal "multipart/related; boundary=b", form_data.content_type
+    assert_instance_of String, form_data.content_type
+  end
+
   def test_content_length
     assert_equal @form_data.to_s.bytesize, @form_data.content_length
   end