diff --git a/lib/rack/test/utils.rb b/lib/rack/test/utils.rb
index d1f74568..aea7193b 100644
--- a/lib/rack/test/utils.rb
+++ b/lib/rack/test/utils.rb
@@ -152,7 +152,6 @@ def build_file_part(buffer, parameter_name, uploaded_file)
 
         # Handle old versions of Capybara::RackTest::Form::NilUploadedFile
         if uploaded_file.respond_to?(:set_encoding)
-          uploaded_file.set_encoding(Encoding::BINARY)
           uploaded_file.append_to(buffer)
         end
 
diff --git a/spec/rack/test/utils_spec.rb b/spec/rack/test/utils_spec.rb
index 090276d3..7771909c 100644
--- a/spec/rack/test/utils_spec.rb
+++ b/spec/rack/test/utils_spec.rb
@@ -78,6 +78,18 @@
 describe 'Rack::Test::Utils.build_multipart' do
   include Rack::Test::Utils
 
+  it 'preserves the source encoding of uploaded StringIO content' do
+    source = +"café"
+    file = Rack::Test::UploadedFile.new(
+      StringIO.new(source),
+      original_filename: 'content.txt'
+    )
+
+    Rack::Test::Utils.build_multipart('file' => file)
+
+    source.encoding.must_equal Encoding::UTF_8
+  end
+
   it 'builds multipart bodies' do
     files = Rack::Test::UploadedFile.new(multipart_file('foo.txt'))
     data  = Rack::Test::Utils.build_multipart('submit-name' => 'Larry', 'files' => files)