diff --git a/app/models/concerns/attachable.rb b/app/models/concerns/attachable.rb
index b2bcbf1b4..a08d0bddc 100644
--- a/app/models/concerns/attachable.rb
+++ b/app/models/concerns/attachable.rb
@@ -18,7 +18,7 @@ module Attachable
               },
               file_size: {
                 less_than_or_equal_to: ->(record) { record.max_file_size.megabytes },
-                if: -> { association_class && attachment.attached? },
+                if: -> { association_class && attachment.attached? && attachment.new_record? },
                 message: ->(record, *) do
                   I18n.t("#{record.model_name.plural}.errors.messages.in_between",
                          min: "0 Bytes",
diff --git a/spec/models/attachable_spec.rb b/spec/models/attachable_spec.rb
index 2cb13825b..3c3c48d14 100644
--- a/spec/models/attachable_spec.rb
+++ b/spec/models/attachable_spec.rb
@@ -14,4 +14,48 @@ describe Attachable do
 
     expect(build(:image).file_path).to include "storage/tenants/image-master/"
   end
+
+  context "file size validation" do
+    it "is not applied when the image attachment has not changed" do
+      image = create(:image, :proposal_image)
+
+      expect(image.valid?).to be(true)
+
+      Setting["uploads.images.max_size"] = 0.1
+
+      expect(image.valid?).to be(true)
+    end
+
+    it "is applied when the image attachment changes" do
+      image = create(:image, :proposal_image)
+
+      expect(image.valid?).to be(true)
+
+      Setting["uploads.images.max_size"] = 0.1
+      image.attachment = Rack::Test::UploadedFile.new("spec/fixtures/files/clippy.png")
+
+      expect(image.valid?).to be(false)
+    end
+
+    it "is not applied when the document attachment has not changed" do
+      document = create(:document, :proposal_document)
+
+      expect(document.valid?).to be(true)
+
+      Setting["uploads.documents.max_size"] = 0.1
+
+      expect(document.valid?).to be(true)
+    end
+
+    it "is applied when the document attachment changes" do
+      document = create(:document, :proposal_document)
+
+      expect(document.valid?).to be(true)
+
+      Setting["uploads.documents.max_size"] = 0.1
+      document.attachment = Rack::Test::UploadedFile.new("spec/fixtures/files/clippy.pdf")
+
+      expect(document.valid?).to be(false)
+    end
+  end
 end