Validate maximum attachment size in application settings

`max_attachment_size` in `ApplicationSetting` should be present,
only integers greater than zero are valid.

Closes #13188

app/models/application_setting.rb

@@ -92,6 +92,10 @@ class ApplicationSetting < ActiveRecord::Base
             presence: true,
             if: :akismet_enabled
 
+  validates :max_attachment_size,
+            presence: true,
+            numericality: { only_integer: true, greater_than: 0 }
+
   validates_each :restricted_visibility_levels do |record, attr, value|
     unless value.nil?
       value.each do |level|

spec/models/application_setting_spec.rb

@@ -66,6 +66,14 @@ describe ApplicationSetting, models: true do
     it { is_expected.to allow_value(http).for(:after_sign_out_path) }
     it { is_expected.to allow_value(https).for(:after_sign_out_path) }
     it { is_expected.not_to allow_value(ftp).for(:after_sign_out_path) }
+
+    it { is_expected.to validate_presence_of(:max_attachment_size) }
+
+    it do
+      is_expected.to validate_numericality_of(:max_attachment_size)
+        .only_integer
+        .is_greater_than(0)
+    end
   end
 
   context 'restricted signup domains' do