image_validator.rb

class ImageValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value =~ /\.(gif|jpg|png)\Z}/i
      record.errors[attribute] << (options[:message] || "is not an image")
    end
  end
end

product.rb

class Product < ActiveRecord::Base
  validates :title, :description, presence: true
  validates :title, uniqueness: true
  validates :price, numericality: { greater_than_or_equal_to: 0.01 }
  validates :image_url, image: true
end

product_spec.rb

require 'rails_helper'

RSpec.describe Product, type: :model do
  it { should validate_presence_of(:title) }
  it { should validate_uniqueness_of(:title) }
  it { should validate_numericality_of(:price)
        .is_greater_than_or_equal_to(0.01) }
  it { should validate_image_of(:image_url)}
end

zimage_validator_spec.rb

require 'spec_helper'

describe "ImageValidator" do
  before(:each) do
    @validator = ImageValidator.new({attributes: {not_blank: true}})
    @model = double('model')
    allow(@model).to receive("errors").and_return([])
    allow(@model.errors).to receive('[]').and_return([])
  end

  it "should validate valid image" do
    expect(@model).not_to receive(:errors)
    @validator.validate_each(@model, :image_url, "image.png")
  end

  it "should validate invalid image" do
    expect(@model).to receive(:errors)
    @validator.validate_each(@model, :image_url, "totally_an_image.csv")
  end
end

RSpec::Matchers.define :validate_image_of do |attribute|
  match do |actual|
    validates_valid = -> {
      actual.send("#{attribute}=", 'image.png')
      actual.valid?
      actual.errors[attribute].empty?
    }.call

    validates_invalid = -> {
      actual.send("#{attribute}=", 'totally_an_image.csv')
      actual.valid?
      actual.errors[attribute].any?
    }.call

    validates_valid && validates_invalid
  end
end