diff --git a/app/controllers/contact_controller.rb b/app/controllers/contact_controller.rb
index 08962de84..f003fa0a0 100644
--- a/app/controllers/contact_controller.rb
+++ b/app/controllers/contact_controller.rb
@@ -2,6 +2,7 @@
 
 class ContactController < ApplicationController
   invisible_captcha only: [:create], on_spam: :redirect_to_root
+  before_action :reject_invalid_attachment, only: [:create]
 
   def index
     @form = ContactForm.new(tags: contact_form_params.fetch(:tags, []), dossier_id: dossier&.id)
@@ -75,4 +76,14 @@ class ContactController < ApplicationController
       params.permit(:dossier_id, tags: []) # prefilling form
     end
   end
+
+  def reject_invalid_attachment
+    piece_jointe = params.dig(:contact_form, :piece_jointe)
+    return if piece_jointe.nil?
+    return if piece_jointe.is_a?(ActionDispatch::Http::UploadedFile)
+
+    @form = ContactForm.new(user: current_user)
+    flash.alert = t('invalid_piece_jointe', scope: "contact.create")
+    render(@form.for_admin ? :admin : :index, status: :unprocessable_entity)
+  end
 end
diff --git a/config/locales/views/contact/en.yml b/config/locales/views/contact/en.yml
index c0815a287..993a65256 100644
--- a/config/locales/views/contact/en.yml
+++ b/config/locales/views/contact/en.yml
@@ -71,3 +71,4 @@ en:
     create:
       direct_message_sent: Your message has been sent to the mailbox in your file.
       message_sent: Your message has been sent.
+      invalid_piece_jointe: 'The attachment must be a file'
diff --git a/config/locales/views/contact/fr.yml b/config/locales/views/contact/fr.yml
index 6d5b3d629..aca1dd861 100644
--- a/config/locales/views/contact/fr.yml
+++ b/config/locales/views/contact/fr.yml
@@ -72,3 +72,4 @@ fr:
     create:
       direct_message_sent: Votre message a été envoyé sur la messagerie de votre dossier.
       message_sent: Votre message a été envoyé.
+      invalid_piece_jointe: 'La pièce jointe doit être un fichier'
diff --git a/spec/controllers/contact_controller_spec.rb b/spec/controllers/contact_controller_spec.rb
index 6191262c5..6f3ea6def 100644
--- a/spec/controllers/contact_controller_spec.rb
+++ b/spec/controllers/contact_controller_spec.rb
@@ -198,6 +198,16 @@ describe ContactController, question_type: :controller do
           expect(response.body).to include("un message")
         end
       end
+
+      context "with an invalid attachment type" do
+        let(:params) { super().merge(piece_jointe: "not_a_file") }
+
+        it "returns unprocessable entity status" do
+          subject
+          expect(response).to have_http_status(:unprocessable_entity)
+          expect(response.body).to include("La pièce jointe doit être un fichier")
+        end
+      end
     end
   end