diff --git a/lib/jsonapi/deserialization.rb b/lib/jsonapi/deserialization.rb
index 0a9d5de..a2593ec 100644
--- a/lib/jsonapi/deserialization.rb
+++ b/lib/jsonapi/deserialization.rb
@@ -35,9 +35,11 @@ def jsonapi_inflector
     def jsonapi_deserialize(document, options = {})
       if document.respond_to?(:permit!)
         # Handle Rails params...
-        primary_data = document.dup.require(:data).permit!.as_json
+        primary_data =
+          document.dup.require(:data).permit!.to_h.deep_stringify_keys
       elsif document.is_a?(Hash)
-        primary_data = (document.as_json['data'] || {}).deep_dup
+        primary_data =
+          (document.to_h.deep_stringify_keys['data'] || {}).deep_dup
       else
         return {}
       end
diff --git a/spec/deserialization_spec.rb b/spec/deserialization_spec.rb
index b23c183..897ad48 100644
--- a/spec/deserialization_spec.rb
+++ b/spec/deserialization_spec.rb
@@ -2,6 +2,7 @@
 
 RSpec.describe JSONAPI::Deserialization do
   let(:jsonapi_deserialize) { UsersController.new.method(:jsonapi_deserialize) }
+  let(:file_object) { ::File.new('./Gemfile') }
   let(:document) do
     {
       data: {
@@ -9,7 +10,8 @@
         type: 'note',
         attributes: {
           title: 'Title 1',
-          date: '2015-12-20'
+          date: '2015-12-20',
+          file: file_object
         },
         relationships: {
           author: {
@@ -40,7 +42,7 @@
 
   describe '#jsonapi_deserialize' do
     it do
-      expect(jsonapi_deserialize.call(document)).to eq(
+      expect(jsonapi_deserialize.call(document)).to include(
         'id' => 1,
         'date' => '2015-12-20',
         'title' => 'Title 1',
@@ -50,6 +52,18 @@
       )
     end
 
+    context 'with file attribute' do
+      it do
+        expect(jsonapi_deserialize.call(document)['file'])
+          .to be_an_instance_of(File)
+      end
+
+      it do
+        expect(jsonapi_deserialize.call(document)['file'])
+          .not_to be_an_instance_of(String)
+      end
+    end
+
     context 'with `only`' do
       it do
         expect(jsonapi_deserialize.call(document, only: :notes)).to eq(
@@ -61,7 +75,7 @@
     context 'with `except`' do
       it do
         expect(
-          jsonapi_deserialize.call(document, except: [:date, :title])
+          jsonapi_deserialize.call(document, except: [:date, :title, :file])
         ).to eq(
           'id' => 1,
           'author_id' => 2,