Add File as valid param type

With a type declaration of `File`, the input parameter hash of the uploaded
file is coerce into a new UploadedFile object which conforms to the
`Rack::Multipart::UploadedFile` interface.

The input parameter hash must be of the shape:
- `filename`: the original file name of the uploaded file
- `head`: the header lines of the multipart request
- `name`: the parameter name
- `tempfile`: the `Tempfile` created from the content of the multipart request
- `type`: the content media/MIME type

README.md

@@ -68,6 +68,7 @@ By declaring parameter types, incoming parameters will automatically be transfor
 * `Array` _("1,2,3,4,5")_
 * `Hash` _(key1:value1,key2:value2)_
 * `Date`, `Time`, & `DateTime`
+* `File`
 
 ### Validations
 

lib/sinatra/param.rb

@@ -1,4 +1,5 @@
 require 'sinatra/base'
+require 'sinatra/param/uploaded_file'
 require 'sinatra/param/version'
 require 'date'
 require 'time'
@@ -121,6 +122,7 @@ def coerce(param, type, options = {})
         return DateTime.parse(param) if type == DateTime
         return Array(param.split(options[:delimiter] || ",")) if type == Array
         return Hash[param.split(options[:delimiter] || ",").map{|c| c.split(options[:separator] || ":")}] if type == Hash
+        return UploadedFile.from_param(param) if type == File
         if [TrueClass, FalseClass, Boolean].include? type
           coerced = /^(false|f|no|n|0)$/i === param.to_s ? false : /^(true|t|yes|y|1)$/i === param.to_s ? true : nil
           raise ArgumentError if coerced.nil?

lib/sinatra/param/uploaded_file.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+module Sinatra
+  module Param
+    # A wrapper/delegator to an uploaded file.
+    #
+    # The attributes are the same as the original parameter hash keys with
+    # extra aliases conforming to the `Rack::Multipart::UploadedFile` interface.
+    class UploadedFile < SimpleDelegator
+      attr_reader :filename, :head, :name, :tempfile, :type
+
+      def initialize(filename:, head:, name:, tempfile:, type:)
+        super(tempfile)
+        @filename = filename
+        @head = head
+        @name = name
+        @tempfile = tempfile
+        @type = type
+      end
+
+      alias content_type type
+      alias original_filename filename
+
+      def self.from_param(param)
+        new(
+          filename: param.fetch(:filename),
+          head: param.fetch(:head),
+          name: param.fetch(:name),
+          tempfile: param.fetch(:tempfile),
+          type: param.fetch(:type),
+        )
+      rescue KeyError, NoMethodError => e
+        raise ArgumentError, e.message
+      end
+    end
+  end
+end

spec/dummy/app.rb

@@ -70,6 +70,21 @@ class App < Sinatra::Base
     params.to_json
   end
 
+  put '/coerce/file' do
+    param :arg, File
+    {
+      arg: {
+        body: params[:arg].read,
+        content_type: params[:arg].content_type,
+        filename: params[:arg].filename,
+        head: params[:arg].head,
+        name: params[:arg].name,
+        original_filename: params[:arg].original_filename,
+        type: params[:arg].type,
+      },
+    }.to_json
+  end
+
   get '/coerce/boolean' do
     param :arg, Boolean
     params.to_json

spec/parameter_type_coercion_spec.rb

@@ -130,6 +130,47 @@
     end
   end
 
+  describe 'File' do
+    let(:arg) { Rack::Test::UploadedFile.new(content, 'text/csv', original_filename: 'file.csv') }
+    let(:content) { StringIO.new('content') }
+    let(:head) do
+      <<~HEAD
+        Content-Disposition: form-data; name="arg"; filename="file.csv"\r
+        Content-Type: text/csv\r
+        Content-Length: 7\r
+      HEAD
+    end
+
+    it 'coerces files' do
+      put('/coerce/file', arg: arg) do |response|
+        expect(response.status).to eql 200
+        parsed_body = JSON.parse(response.body)
+        expect(parsed_body['arg']).to be_a(Hash)
+        expect(parsed_body['arg']).to eq({
+          'body' => 'content',
+          'content_type' => 'text/csv',
+          'filename' => 'file.csv',
+          'head' => head,
+          'name' => 'arg',
+          'original_filename' => 'file.csv',
+          'type' => 'text/csv',
+        })
+      end
+    end
+
+    it 'returns 400 when not a hash' do
+      put('/coerce/file', arg: 'arg') do |response|
+        expect(response.status).to eql 400
+      end
+    end
+
+    it 'returns 400 when not a file upload hash' do
+      put('/coerce/file', arg: { 'a' => 'a' }) do |response|
+        expect(response.status).to eql 400
+      end
+    end
+  end
+
   describe 'Boolean' do
     it 'coerces truthy booleans to true' do
       %w(1 true t yes y).each do |bool|