Итак, наконец-то я получил решение.
Я добавил 2 условных проверки для того же
class Media < ActiveRecord::Base
belongs_to :memory
validates_attachment_presence :source
validates_attachment_content_type :source,
:content_type => ['video/mp4'],
:message => "Sorry, right now we only support MP4 video",
:if => :is_type_of_video?
validates_attachment_content_type :source,
:content_type => ['image/png', 'image/jpeg', 'image/jpg', 'image/gif'],
:message => "Different error message",
:if => :is_type_of_image?
has_attached_file :source
protected
def is_type_of_video?
source.content_type =~ %r(video)
end
def is_type_of_image?
source.content_type =~ %r(image)
end
end