diff --git a/.rubocop.yml b/.rubocop.yml
index 2cab121..0c57a91 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -28,6 +28,10 @@ Metrics/BlockLength:
     - spec/**/*
     - rack-test.gemspec
 
+# Rationale: Enforcing maximum module length makes code worse, without exception
+Metrics/ModuleLength:
+  Enabled: false
+
 # Rationale: allow Weirich-style blocks, but do not enforce them.
 Style/BlockDelimiters:
   Enabled: false
diff --git a/lib/rack/test/uploaded_file.rb b/lib/rack/test/uploaded_file.rb
index 7e9b5ca..925b7e5 100644
--- a/lib/rack/test/uploaded_file.rb
+++ b/lib/rack/test/uploaded_file.rb
@@ -49,6 +49,21 @@ def method_missing(method_name, *args, &block) #:nodoc:
         tempfile.public_send(method_name, *args, &block)
       end
 
+      # Append to given buffer in 64K chunks to avoid multiple large
+      # copies of file data in memory.  Rewind tempfile before and
+      # after to make sure all data in tempfile is appended to the
+      # buffer.
+      def append_to(buffer)
+        tempfile.rewind
+
+        buf = String.new
+        buffer << tempfile.readpartial(65_536, buf) until tempfile.eof?
+
+        tempfile.rewind
+
+        nil
+      end
+
       def respond_to_missing?(method_name, include_private = false) #:nodoc:
         tempfile.respond_to?(method_name, include_private) || super
       end
diff --git a/lib/rack/test/utils.rb b/lib/rack/test/utils.rb
index 44d3dea..4cd48b3 100644
--- a/lib/rack/test/utils.rb
+++ b/lib/rack/test/utils.rb
@@ -125,14 +125,16 @@ def build_primitive_part(parameter_name, value)
 
       def build_file_part(parameter_name, uploaded_file)
         uploaded_file.set_encoding(Encoding::BINARY) if uploaded_file.respond_to?(:set_encoding)
-        <<-EOF
+        buffer = String.new
+        buffer << (<<-EOF)
 --#{MULTIPART_BOUNDARY}\r
 Content-Disposition: form-data; name="#{parameter_name}"; filename="#{escape_path(uploaded_file.original_filename)}"\r
 Content-Type: #{uploaded_file.content_type}\r
 Content-Length: #{uploaded_file.size}\r
 \r
-#{uploaded_file.read}\r
 EOF
+        uploaded_file.append_to(buffer)
+        buffer << "\r\n"
       end
       module_function :build_file_part
     end
diff --git a/spec/rack/test/utils_spec.rb b/spec/rack/test/utils_spec.rb
index a841017..6429669 100644
--- a/spec/rack/test/utils_spec.rb
+++ b/spec/rack/test/utils_spec.rb
@@ -73,7 +73,8 @@
       params = Rack::Multipart.parse_multipart(env)
       check expect(params['submit-name']).to eq('Larry')
       check expect(params['files'][:filename]).to eq('foo.txt')
-      expect(params['files'][:tempfile].read).to eq("bar\n")
+      expect(files.pos).to eq(0)
+      expect(params['files'][:tempfile].read).to eq(files.read)
     end
 
     it 'builds multipart bodies from array of files' do