diff --git a/Gemfile b/Gemfile
index 900e882a..56bd0f34 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,6 +6,7 @@ gemspec
 # Development dependencies. Not included in the publised gem.
 gem "byebug", "~> 11.1.3"
 gem "dotenv", "~> 2.8.1" # >= v3 will require removing support for Ruby 2.7 from CI.
+gem "mime-types", "~> 3.5"
 gem "rake", "~> 13.2.1"
 gem "rspec", "~> 3.13"
 gem "rubocop", "~> 1.74.0"
diff --git a/Gemfile.lock b/Gemfile.lock
index 2b19c222..4361f6d5 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -32,6 +32,9 @@ GEM
     json (2.10.2)
     language_server-protocol (3.17.0.4)
     lint_roller (1.1.0)
+    mime-types (3.5.2)
+      mime-types-data (~> 3.2015)
+    mime-types-data (3.2024.0604)
     multipart-post (2.3.0)
     parallel (1.26.3)
     parser (3.3.7.1)
@@ -87,6 +90,7 @@ PLATFORMS
 DEPENDENCIES
   byebug (~> 11.1.3)
   dotenv (~> 2.8.1)
+  mime-types (~> 3.5)
   rake (~> 13.2.1)
   rspec (~> 3.13)
   rubocop (~> 1.74.0)
diff --git a/lib/openai.rb b/lib/openai.rb
index 4c29c24a..6ea30b91 100644
--- a/lib/openai.rb
+++ b/lib/openai.rb
@@ -21,6 +21,7 @@
 require_relative "openai/version"
 require_relative "openai/batches"
 require_relative "openai/usage"
+require_relative "openai/mime_types"
 
 module OpenAI
   class Error < StandardError; end
diff --git a/lib/openai/http.rb b/lib/openai/http.rb
index 5459f64c..3f30d710 100644
--- a/lib/openai/http.rb
+++ b/lib/openai/http.rb
@@ -87,11 +87,9 @@ def multipart_parameters(parameters)
         # Faraday::UploadIO does not require a path, so we will pass it
         # only if it is available. This allows StringIO objects to be
         # passed in as well.
-        path = value.respond_to?(:path) ? value.path : nil
-        # Doesn't seem like OpenAI needs mime_type yet, so not worth
-        # the library to figure this out. Hence the empty string
-        # as the second argument.
-        Faraday::UploadIO.new(value, "", path)
+        return Faraday::UploadIO.new(value, "", nil) unless value.respond_to?(:path)
+
+        Faraday::UploadIO.new(value, mime_type_for(value.path), value.path)
       end
     end
 
@@ -108,5 +106,9 @@ def configure_json_post_request(req, parameters)
       req.headers = headers
       req.body = req_parameters.to_json
     end
+
+    def mime_type_for(path)
+      MIME::Types.of(path).first.to_s
+    end
   end
 end
diff --git a/lib/openai/mime_types.rb b/lib/openai/mime_types.rb
new file mode 100644
index 00000000..c741b3c4
--- /dev/null
+++ b/lib/openai/mime_types.rb
@@ -0,0 +1,17 @@
+require "mime/types"
+
+module OpenAI
+  module MimeTypes
+    REGISTER_MIME_TYPES = { "jsonl" => "application/json" }.freeze
+
+    def self.register
+      REGISTER_MIME_TYPES.each do |register_type, mime_type|
+        next if MIME::Types.of(register_type).any?
+
+        MIME::Types[mime_type].first.add_extensions(register_type)
+      end
+    end
+  end
+end
+
+OpenAI::MimeTypes.register