Validate attribute only if it present (only if user fill in it)
Solution 1:
Add :allow_blank => true and it should do what you want.
Solution 2:
Maybe :if => lambda {|attr| attr.present?} will help?
Solution 3:
Some validations accept the options :allow_blank => true or :allow_nil => true.
If this fails, use :if condition, like this:
validates_format_of :avatar_file_name,
:with=>/\.(jpeg|jpg|png|gif)$/i,
:message=> I18n.t("..."),
:on => :update,
:if => lambda{ |object| object.avatar_file_name.present? }
But i encourage you to use allows. Much cleaner.