diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index 59acac47..0ac3c2a5 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "0.27.0"
+  ".": "0.27.1"
 }
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
index e467bd35..4dc9e9da 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,13 @@
 # Changelog
 
+## 0.27.1 (2025-09-29)
+
+Full Changelog: [v0.27.0...v0.27.1](https://github.com/openai/openai-ruby/compare/v0.27.0...v0.27.1)
+
+### Bug Fixes
+
+* always send `filename=...` for multipart requests where a file is expected ([99849fd](https://github.com/openai/openai-ruby/commit/99849fddf478869b57b77cbe208efd13d9a8246e))
+
 ## 0.27.0 (2025-09-26)
 
 Full Changelog: [v0.26.0...v0.27.0](https://github.com/openai/openai-ruby/compare/v0.26.0...v0.27.0)
diff --git a/Gemfile.lock b/Gemfile.lock
index 00a44544..a6273fa9 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -11,7 +11,7 @@ GIT
 PATH
   remote: .
   specs:
-    openai (0.27.0)
+    openai (0.27.1)
       connection_pool
 
 GEM
diff --git a/README.md b/README.md
index 7a05c316..642344a2 100644
--- a/README.md
+++ b/README.md
@@ -15,7 +15,7 @@ To use this gem, install via Bundler by adding the following to your application
 <!-- x-release-please-start-version -->
 
 ```ruby
-gem "openai", "~> 0.27.0"
+gem "openai", "~> 0.27.1"
 ```
 
 <!-- x-release-please-end -->
diff --git a/lib/openai/file_part.rb b/lib/openai/file_part.rb
index f853ca4d..da661b25 100644
--- a/lib/openai/file_part.rb
+++ b/lib/openai/file_part.rb
@@ -38,18 +38,21 @@ def to_json(*a) = read.to_json(*a)
     def to_yaml(*a) = read.to_yaml(*a)
 
     # @param content [Pathname, StringIO, IO, String]
-    # @param filename [String, nil]
+    # @param filename [Pathname, String, nil]
     # @param content_type [String, nil]
     def initialize(content, filename: nil, content_type: nil)
-      @content = content
+      @content_type = content_type
       @filename =
-        case content
-        in Pathname
-          filename.nil? ? content.basename.to_path : ::File.basename(filename)
+        case [filename, (@content = content)]
+        in [String | Pathname, _]
+          ::File.basename(filename)
+        in [nil, Pathname]
+          content.basename.to_path
+        in [nil, IO]
+          content.to_path
         else
-          filename.nil? ? nil : ::File.basename(filename)
+          filename
         end
-      @content_type = content_type
     end
   end
 end
diff --git a/lib/openai/internal/type/file_input.rb b/lib/openai/internal/type/file_input.rb
index a5dcdef7..194cd38b 100644
--- a/lib/openai/internal/type/file_input.rb
+++ b/lib/openai/internal/type/file_input.rb
@@ -82,17 +82,20 @@ def coerce(value, state:)
           #
           # @return [Pathname, StringIO, IO, String, Object]
           def dump(value, state:)
-            # rubocop:disable Lint/DuplicateBranch
             case value
+            in StringIO | String
+              # https://datatracker.ietf.org/doc/html/rfc7578#section-4.2
+              # while not required, a filename is recommended, and in practice many servers do expect this
+              OpenAI::FilePart.new(value, filename: "upload")
             in IO
               state[:can_retry] = false
+              value.to_path.nil? ? OpenAI::FilePart.new(value, filename: "upload") : value
             in OpenAI::FilePart if value.content.is_a?(IO)
               state[:can_retry] = false
+              value
             else
+              value
             end
-            # rubocop:enable Lint/DuplicateBranch
-
-            value
           end
 
           # @api private
diff --git a/lib/openai/version.rb b/lib/openai/version.rb
index 472922d8..c1472d01 100644
--- a/lib/openai/version.rb
+++ b/lib/openai/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module OpenAI
-  VERSION = "0.27.0"
+  VERSION = "0.27.1"
 end
diff --git a/rbi/openai/file_part.rbi b/rbi/openai/file_part.rbi
index 5df7359c..07a8b7a6 100644
--- a/rbi/openai/file_part.rbi
+++ b/rbi/openai/file_part.rbi
@@ -27,7 +27,7 @@ module OpenAI
     sig do
       params(
         content: T.any(Pathname, StringIO, IO, String),
-        filename: T.nilable(String),
+        filename: T.nilable(T.any(Pathname, String)),
         content_type: T.nilable(String)
       ).returns(T.attached_class)
     end
diff --git a/sig/openai/file_part.rbs b/sig/openai/file_part.rbs
index c517bee8..73892c6a 100644
--- a/sig/openai/file_part.rbs
+++ b/sig/openai/file_part.rbs
@@ -14,7 +14,7 @@ module OpenAI
 
     def initialize: (
       Pathname | StringIO | IO | String content,
-      ?filename: String?,
+      ?filename: (Pathname | String)?,
       ?content_type: String?
     ) -> void
   end
diff --git a/test/openai/internal/util_test.rb b/test/openai/internal/util_test.rb
index 26bacf49..786e7c45 100644
--- a/test/openai/internal/util_test.rb
+++ b/test/openai/internal/util_test.rb
@@ -227,20 +227,24 @@ def test_encoding_length
 
   def test_file_encode
     file = Pathname(__FILE__)
+    fileinput = OpenAI::Internal::Type::Converter.dump(OpenAI::Internal::Type::FileInput, "abc")
     headers = {"content-type" => "multipart/form-data"}
     cases = {
-      "abc" => "abc",
-      StringIO.new("abc") => "abc",
-      OpenAI::FilePart.new("abc") => "abc",
-      OpenAI::FilePart.new(StringIO.new("abc")) => "abc",
-      file => /^class OpenAI/,
-      OpenAI::FilePart.new(file) => /^class OpenAI/
+      "abc" => ["", "abc"],
+      StringIO.new("abc") => ["", "abc"],
+      fileinput => %w[upload abc],
+      OpenAI::FilePart.new(StringIO.new("abc")) => ["", "abc"],
+      file => [file.basename.to_path, /^class OpenAI/],
+      OpenAI::FilePart.new(file, filename: "d o g") => ["d%20o%20g", /^class OpenAI/]
     }
-    cases.each do |body, val|
+    cases.each do |body, testcase|
+      filename, val = testcase
       encoded = OpenAI::Internal::Util.encode_content(headers, body)
       cgi = FakeCGI.new(*encoded)
+      io = cgi[""]
       assert_pattern do
-        cgi[""].read => ^val
+        io.original_filename => ^filename
+        io.read => ^val
       end
     end
   end
@@ -261,7 +265,14 @@ def test_hash_encode
       cgi = FakeCGI.new(*encoded)
       testcase.each do |key, val|
         assert_pattern do
-          cgi[key] => ^val
+          parsed =
+            case (p = cgi[key])
+            in StringIO
+              p.read
+            else
+              p
+            end
+          parsed => ^val
         end
       end
     end