diff --git a/lib/anthropic/internal/util.rb b/lib/anthropic/internal/util.rb
index d33935ba..f2dd310d 100644
--- a/lib/anthropic/internal/util.rb
+++ b/lib/anthropic/internal/util.rb
@@ -550,14 +550,16 @@ def encode_query_params(query)
               y,
               val: val.content,
               closing: closing,
-              content_type: val.content_type
+              content_type: val.content_type || infer_multipart_content_type(val.filename)
             )
           in Pathname
+            content_type ||= infer_multipart_content_type(val)
             y << format(content_line, content_type || "application/octet-stream")
             io = val.open(binmode: true)
             closing << io.method(:close)
             IO.copy_stream(io, y)
           in IO
+            content_type ||= infer_multipart_content_type(val.to_path)
             y << format(content_line, content_type || "application/octet-stream")
             IO.copy_stream(val, y)
           in StringIO
@@ -573,6 +575,20 @@ def encode_query_params(query)
           y << "\r\n"
         end
 
+        # @api private
+        #
+        # @param filename [Pathname, String, nil]
+        #
+        # @return [String, nil]
+        private def infer_multipart_content_type(filename)
+          case ::File.extname(filename.to_s).downcase
+          when ".pdf"
+            "application/pdf"
+          when ".txt"
+            "text/plain"
+          end
+        end
+
         # @api private
         #
         # @param y [Enumerator::Yielder]
diff --git a/test/anthropic/internal/util_test.rb b/test/anthropic/internal/util_test.rb
index 133445e3..353e4aa9 100644
--- a/test/anthropic/internal/util_test.rb
+++ b/test/anthropic/internal/util_test.rb
@@ -278,6 +278,26 @@ def test_multipart_filename_quoting
     refute_includes(body, "\r\nEvil:")
   end
 
+  def test_multipart_document_content_type_inference
+    headers = {"content-type" => "multipart/form-data"}
+    cases = {
+      Anthropic::FilePart.new(StringIO.new("x"), filename: "document.pdf") => "application/pdf",
+      Anthropic::FilePart.new(StringIO.new("x"), filename: "document.txt") => "text/plain",
+      Anthropic::FilePart.new(
+        StringIO.new("x"),
+        filename: "document.pdf",
+        content_type: "application/custom"
+      ) => "application/custom"
+    }
+
+    cases.each do |file, content_type|
+      _headers, stream = Anthropic::Internal::Util.encode_content(headers, {file: file})
+      body = stream.respond_to?(:read) ? stream.read : stream.to_a.join
+
+      assert_includes(body, "Content-Type: #{content_type}\r\n\r\n")
+    end
+  end
+
   def test_hash_encode
     headers = {"content-type" => "multipart/form-data"}
     cases = {