diff --git a/lib/context_dev/file_part.rb b/lib/context_dev/file_part.rb
index 58d885e4..36f6fb12 100644
--- a/lib/context_dev/file_part.rb
+++ b/lib/context_dev/file_part.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "pathname"
+
 module ContextDev
   class FilePart
     # @return [Pathname, StringIO, IO, String]
@@ -49,7 +51,7 @@ def initialize(content, filename: nil, content_type: nil)
         in [nil, Pathname]
           content.basename.to_path
         in [nil, IO]
-          content.to_path
+          content.to_path&.then { ::File.basename(_1) }
         else
           filename
         end
diff --git a/lib/context_dev/internal/util.rb b/lib/context_dev/internal/util.rb
index 1c922103..3650540a 100644
--- a/lib/context_dev/internal/util.rb
+++ b/lib/context_dev/internal/util.rb
@@ -592,7 +592,10 @@ def encode_query_params(query)
           in ContextDev::FilePart unless val.filename.nil?
             filename = encode_path(val.filename)
             y << "; filename=\"#{filename}\""
-          in Pathname | IO
+          in Pathname
+            filename = encode_path(::File.basename(val.to_path))
+            y << "; filename=\"#{filename}\""
+          in IO unless val.to_path.nil?
             filename = encode_path(::File.basename(val.to_path))
             y << "; filename=\"#{filename}\""
           else
diff --git a/test/context_dev/file_part_test.rb b/test/context_dev/file_part_test.rb
index a922b3d2..63fcea59 100644
--- a/test/context_dev/file_part_test.rb
+++ b/test/context_dev/file_part_test.rb
@@ -10,4 +10,19 @@ def test_to_json
     assert_equal(text.to_json, filepart.to_json)
     assert_equal(text.to_yaml, filepart.to_yaml)
   end
+
+  def test_filename_basename_for_io
+    file = File.open(__FILE__)
+    filepart = ContextDev::FilePart.new(file)
+    assert_equal(File.basename(__FILE__), filepart.filename)
+    file.close
+  end
+
+  def test_pathless_io
+    r, w = IO.pipe
+    filepart = ContextDev::FilePart.new(r)
+    assert_nil(filepart.filename)
+    r.close
+    w.close
+  end
 end
diff --git a/test/context_dev/internal/util_test.rb b/test/context_dev/internal/util_test.rb
index 1dfca4d2..226c076b 100644
--- a/test/context_dev/internal/util_test.rb
+++ b/test/context_dev/internal/util_test.rb
@@ -259,6 +259,21 @@ def test_file_encode
     end
   end
 
+  def test_pathless_io_encode
+    r, w = IO.pipe
+    w.write("streamed data")
+    w.close
+    headers = {"content-type" => "multipart/form-data"}
+    encoded = ContextDev::Internal::Util.encode_content(headers, r)
+    cgi = FakeCGI.new(*encoded)
+    io = cgi[""]
+    assert_pattern do
+      io.original_filename => ""
+      io.read => "streamed data"
+    end
+    r.close
+  end
+
   def test_hash_encode
     headers = {"content-type" => "multipart/form-data"}
     cases = {