[linter] Validate screenshot URLs return a valid image

lib/cocoapods/validator.rb

@@ -238,6 +238,8 @@ module Pod
       if resp && !resp.success?
         warning "The URL (#{url}) is not reachable."
       end
+
+      resp
     end
 
     # Performs validations related to the `homepage` attribute.
@@ -252,7 +254,10 @@ module Pod
     #
     def validate_screenshots(spec)
       spec.screenshots.each do |screenshot|
-        validate_url(screenshot)
+        request = validate_url(screenshot)
+        if request && !(request.headers['content-type'] && request.headers['content-type'].first =~ /image\/.*/i)
+          warning "The screenshot #{screenshot} is not a valid image."
+        end
       end
     end
 

spec/unit/validator_spec.rb

@@ -121,20 +121,20 @@ module Pod
           @sut.stubs(:check_file_patterns)
           @sut.stubs(:tear_down_validation_environment)
           @sut.stubs(:validate_homepage)
+          WebMock::API.stub_request(:head, 'banana-corp.local/valid-image.png').to_return(:status => 200, :headers => { 'Content-Type' => 'image/png' })
         end
 
         it "checks if the screenshots are valid" do
-          WebMock::API.stub_request(:head, /banana-corp.local/).to_return(:status => 200)
-          Specification.any_instance.stubs(:screenshots).returns(['http://banana-corp.local/first.png', 'http://banana-corp.local/second.png'])
+          Specification.any_instance.stubs(:screenshots).returns(['http://banana-corp.local/valid-image.png'])
           @sut.validate
           @sut.results.should.be.empty?
         end
 
-        it "should fail if any of the screenshots is not valid" do
-          WebMock::API.stub_request(:head, /not-found/).to_raise(SocketError)
-          Specification.any_instance.stubs(:screenshots).returns(['http://banana-corp.local/first.png', 'http://banana-corp.local/not-found/second.png'])
+        it "should fail if any of the screenshots URLS do not return an image" do
+          WebMock::API.stub_request(:head, 'banana-corp.local/').to_return(:status => 200)
+          Specification.any_instance.stubs(:screenshots).returns(['http://banana-corp.local/valid-image.png', 'http://banana-corp.local/'])
           @sut.validate
-          @sut.results.map(&:to_s).first.should.match /There was a problem validating the URL/
+          @sut.results.map(&:to_s).first.should.match /The screenshot .* is not a valid image/
         end
       end