Always provide format with attachment URLs

When linking to an attachment without the format, the browser does not
necessarily know what to do with the file. Adding the suffix by default
makes that better: Firefox won't, for example, try rendering a PDF file
as HTML.

app/models/alchemy/attachment/url.rb

@@ -23,6 +23,7 @@ def initialize(attachment)
       # @return [String]
       #
       def call(options = {})
+        options[:format] ||= @attachment.suffix
         if options.delete(:download)
           routes.download_attachment_path(@attachment, options)
         else

spec/models/alchemy/attachment_spec.rb

@@ -63,14 +63,14 @@ module Alchemy
         let(:attachment) { create(:alchemy_attachment) }
 
         it "returns local path" do
-          is_expected.to eq "/attachment/#{attachment.id}/show"
+          is_expected.to eq "/attachment/#{attachment.id}/show.png"
         end
 
         context "with download enabled" do
           subject { attachment.url(download: true) }
 
           it "returns local download path" do
-            is_expected.to eq "/attachment/#{attachment.id}/download"
+            is_expected.to eq "/attachment/#{attachment.id}/download.png"
           end
 
           context "with extra params given" do
@@ -88,7 +88,7 @@ module Alchemy
           subject { attachment.url(download: false) }
 
           it "returns local path" do
-            is_expected.to eq "/attachment/#{attachment.id}/show"
+            is_expected.to eq "/attachment/#{attachment.id}/show.png"
           end
 
           context "with extra params given" do