diff --git a/lib/http/form_data.rb b/lib/http/form_data.rb
index 56f95f7..be542bc 100644
--- a/lib/http/form_data.rb
+++ b/lib/http/form_data.rb
@@ -70,8 +70,8 @@ def ensure_hash(obj)
       # @return [Boolean]
       def multipart?(data)
         data.any? do |_, v|
-          next true if v.is_a? FormData::File
-          v.respond_to?(:to_ary) && v.to_ary.any? { |e| e.is_a? FormData::File }
+          next true if v.is_a? FormData::Part
+          v.respond_to?(:to_ary) && v.to_ary.any? { |e| e.is_a? FormData::Part }
         end
       end
     end