diff --git a/app/models/champs/piece_justificative_champ.rb b/app/models/champs/piece_justificative_champ.rb
index 0a3526a07..8fcf58027 100644
--- a/app/models/champs/piece_justificative_champ.rb
+++ b/app/models/champs/piece_justificative_champ.rb
@@ -1,7 +1,8 @@
 class Champs::PieceJustificativeChamp < Champ
-  PIECE_JUSTIFICATIVE_FILE_MAX_SIZE = 200.megabytes
+  MAX_SIZE = 200.megabytes
 
-  PIECE_JUSTIFICATIVE_FILE_ACCEPTED_FORMATS = [
+  ACCEPTED_FORMATS = [
+    "text/plain",
     "application/pdf",
     "application/msword",
     "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
@@ -16,6 +17,13 @@ class Champs::PieceJustificativeChamp < Champ
     "image/jpeg"
   ]
 
+  # TODO: once we're running on Rails 6, re-enable this validation.
+  # See https://github.com/betagouv/demarches-simplifiees.fr/issues/4926
+  #
+  # validates :piece_justificative_file,
+  #   content_type: ACCEPTED_FORMATS,
+  #   size: { less_than: MAX_SIZE }
+
   def main_value_name
     :piece_justificative_file
   end
diff --git a/config/locales/active_storage_validations.fr.yml b/config/locales/active_storage_validations.fr.yml
new file mode 100644
index 000000000..1494483b0
--- /dev/null
+++ b/config/locales/active_storage_validations.fr.yml
@@ -0,0 +1,4 @@
+fr:
+  errors:
+    messages:
+      content_type_invalid: n’est pas d’un type accepté
diff --git a/config/locales/models/champs/fr.yml b/config/locales/models/champs/fr.yml
index 9d9e6b2f0..7a0730a32 100644
--- a/config/locales/models/champs/fr.yml
+++ b/config/locales/models/champs/fr.yml
@@ -3,3 +3,4 @@ fr:
     attributes:
       champ:
         value: La valeur du champ
+        piece_justificative_file: La pièce justificative
diff --git a/spec/models/champs/piece_justificative_champ_spec.rb b/spec/models/champs/piece_justificative_champ_spec.rb
index 506eba015..c2f691884 100644
--- a/spec/models/champs/piece_justificative_champ_spec.rb
+++ b/spec/models/champs/piece_justificative_champ_spec.rb
@@ -1,4 +1,19 @@
+require 'active_storage_validations/matchers'
+
 describe Champs::PieceJustificativeChamp do
+  include ActiveStorageValidations::Matchers
+
+  # TODO: once we're running on Rails 6, re-enable the PieceJustificativeChamp validator,
+  # and re-enable this spec.
+  #
+  # See https://github.com/betagouv/demarches-simplifiees.fr/issues/4926
+  describe "validations", pending: true do
+    subject(:champ_pj) { build(:champ_piece_justificative) }
+
+    it { is_expected.to validate_size_of(:piece_justificative_file).less_than(Champs::PieceJustificativeChamp::MAX_SIZE) }
+    it { is_expected.to validate_content_type_of(:piece_justificative_file).allowing(Champs::PieceJustificativeChamp::ACCEPTED_FORMATS) }
+  end
+
   describe '#for_api' do
     let(:champ_pj) { create(:champ_piece_justificative) }
     let(:metadata) { champ_pj.piece_justificative_file.blob.metadata }