diff --git a/README.md b/README.md
index e6ff597..de3fb07 100644
--- a/README.md
+++ b/README.md
@@ -111,9 +111,9 @@ session = client.session('session_id')
 # Send a message to the session
 response = session.send_message('How do I start?')
 
-# Upload files to the session
-files = [File.open('path/to/file.txt')]
-upload_response = session.upload_files(files)
+# Upload file
+file = File.open('path/to/file.txt')
+upload_response = client.attachment.upload_file(file)
 
 # Update session tags
 session.update_tags(['web', 'app'])
diff --git a/lib/devin_api/client.rb b/lib/devin_api/client.rb
index 4bc03f0..4d2f917 100644
--- a/lib/devin_api/client.rb
+++ b/lib/devin_api/client.rb
@@ -53,10 +53,16 @@ def get(path, params = {})
     # Executes a POST request
     # @param [String] path The path to request
     # @param [Hash] params Body parameters
-    # @return [Hash] Response body
+    # @return [Hash, String] Response body
     def post(path, params = {})
       response = connection.post(path) do |req|
-        req.body = params.to_json
+        if params.values.any? { |v| v.is_a?(Faraday::UploadIO) }
+          req.headers['Content-Type'] = 'multipart/form-data'
+          req.body = params
+        else
+          req.headers['Content-Type'] = 'application/json'
+          req.body = params.to_json
+        end
       end
       parse_response(response)
     end
@@ -93,8 +99,8 @@ def build_connection
         conn.response :json, content_type: /\bjson$/
 
         # Request middlewares
-        conn.request :json
         conn.request :multipart
+        conn.request :json
 
         # Authentication
         conn.headers['Authorization'] = "Bearer #{config.access_token}"
diff --git a/lib/devin_api/endpoints/attachment.rb b/lib/devin_api/endpoints/attachment.rb
index 0a603c2..76adc5f 100644
--- a/lib/devin_api/endpoints/attachment.rb
+++ b/lib/devin_api/endpoints/attachment.rb
@@ -8,13 +8,9 @@ module Attachment
       # @see https://docs.devin.ai/api-reference/attachments/upload-files-for-devin-to-work-with
       #
       # @param [File] file File object to upload
-      # @return [Hash] Response body
+      # @return [String] URL where the uploaded file can be accessed
       def upload_file(file)
-        payload = Faraday::UploadIO.new(
-          file.path,
-          file.content_type || 'application/octet-stream',
-          File.basename(file.path)
-        )
+        payload = { file: Faraday::UploadIO.new(file.path, 'application/octet-stream', File.basename(file.path)) }
 
         connection.post('/v1/attachments', payload).body
       end
diff --git a/lib/devin_api/resources/attachment.rb b/lib/devin_api/resources/attachment.rb
index 89d3a42..30b7daa 100644
--- a/lib/devin_api/resources/attachment.rb
+++ b/lib/devin_api/resources/attachment.rb
@@ -10,12 +10,13 @@ def path
         '/v1/attachments'
       end
 
+      # Upload a file for Devin to work with during sessions
+      # @see https://docs.devin.ai/api-reference/attachments/upload-files-for-devin-to-work-with
+      #
+      # @param [File] file File object to upload
+      # @return [String] URL where the uploaded file can be accessed
       def upload_file(file)
-        payload = Faraday::UploadIO.new(
-          file.path,
-          file.content_type || 'application/octet-stream',
-          File.basename(file.path)
-        )
+        payload = { file: Faraday::UploadIO.new(file.path, 'application/octet-stream', File.basename(file.path)) }
 
         client.connection.post(path, payload).body
       end
diff --git a/spec/core/endpoints/attachment_spec.rb b/spec/core/endpoints/attachment_spec.rb
index d20b252..0709cd1 100644
--- a/spec/core/endpoints/attachment_spec.rb
+++ b/spec/core/endpoints/attachment_spec.rb
@@ -6,23 +6,23 @@
   subject { client }
 
   describe '#upload_file' do
-    let(:file) { double('File', path: '/tmp/test.txt', content_type: 'text/plain') }
+    let(:file) { double('File', path: '/tmp/test.txt') }
+    let(:upload_io) { double('Faraday::UploadIO') }
 
     before do
       allow(File).to receive(:basename).with('/tmp/test.txt').and_return('test.txt')
-      allow(Faraday::UploadIO).to receive(:new).with('/tmp/test.txt', 'text/plain', 'test.txt')
-                                               .and_return('file_content')
+      allow(Faraday::UploadIO).to receive(:new)
+        .with('/tmp/test.txt', 'application/octet-stream', 'test.txt')
+        .and_return(upload_io)
 
-      allow(subject.connection).to receive(:post).with('/v1/attachments', 'file_content').and_return(
-        double('Response', body: { 'file_id' => 'file-123', 'filename' => 'test.txt' })
-      )
+      allow(subject.connection).to receive(:post)
+        .with('/v1/attachments', { file: upload_io })
+        .and_return(double('Response', body: 'https://example.com/file-123'))
     end
 
     it 'should upload a file' do
       response = subject.upload_file(file)
-      expect(response).to be_a(Hash)
-      expect(response['file_id']).to eq('file-123')
-      expect(response['filename']).to eq('test.txt')
+      expect(response).to eq('https://example.com/file-123')
     end
   end
 end
diff --git a/spec/core/resources/attachment_spec.rb b/spec/core/resources/attachment_spec.rb
index a58ee37..113a346 100644
--- a/spec/core/resources/attachment_spec.rb
+++ b/spec/core/resources/attachment_spec.rb
@@ -24,23 +24,23 @@
   end
 
   describe '#upload_file' do
-    let(:file) { double('File', path: '/tmp/test.txt', content_type: 'text/plain') }
+    let(:file) { double('File', path: '/tmp/test.txt') }
+    let(:upload_io) { double('Faraday::UploadIO') }
 
     before do
       allow(File).to receive(:basename).with('/tmp/test.txt').and_return('test.txt')
-      allow(Faraday::UploadIO).to receive(:new).with('/tmp/test.txt', 'text/plain', 'test.txt')
-                                               .and_return('file_content')
+      allow(Faraday::UploadIO).to receive(:new)
+        .with('/tmp/test.txt', 'application/octet-stream', 'test.txt')
+        .and_return(upload_io)
 
-      allow(client.connection).to receive(:post).with('/v1/attachments', 'file_content').and_return(
-        double('Response', body: { 'id' => 'file-123', 'filename' => 'test.txt' })
-      )
+      allow(client.connection).to receive(:post)
+        .with('/v1/attachments', { file: upload_io })
+        .and_return(double('Response', body: 'https://example.com/file-123'))
     end
 
     it 'should upload a file' do
       response = subject.upload_file(file)
-      expect(response).to be_a(Hash)
-      expect(response['id']).to eq('file-123')
-      expect(response['filename']).to eq('test.txt')
+      expect(response).to eq('https://example.com/file-123')
     end
   end
 end