(epub): Avoid duplicate IDs on Package Definition

Currently the ID generation for static files is based on the file name,
but the JS/CSS assets included by ExDoc have the same prefix, example:

* `epub-<hash>.js`
* `epub-<hash>.css`

This will produce duplicate IDs in the Package Document Definition,
which is an error.

Now we also take into account the extension for the static files.

lib/ex_doc/formatter/epub/templates.ex

@@ -14,6 +14,16 @@ defmodule ExDoc.Formatter.EPUB.Templates do
     module_template(config, module_node, summary)
   end
 
+  @doc """
+  Generated ID for static file
+  """
+  def static_file_to_id(static_file) do
+    prefix = static_file |> HTML.filename_to_title() |> HTML.text_to_id()
+    extension = static_file |> Path.extname() |> String.replace_prefix(".", "-")
+
+    "#{prefix}#{extension}"
+  end
+
   @doc """
   Creates the Package Document Definition.
 

lib/ex_doc/formatter/epub/templates/content_template.eex

@@ -28,7 +28,7 @@
       <% end %>
     <% end %>
     <%= for static_file <- static_files do %>
-      <item id="<%= static_file |> HTML.filename_to_title() |> HTML.text_to_id() %>" href="<%= static_file %>" media-type="<%= media_type(Path.extname(static_file)) %>"/>
+      <item id="<%= static_file_to_id(static_file) %>" href="<%= static_file %>" media-type="<%= media_type(Path.extname(static_file)) %>"/>
     <% end %>
     <%= if config.cover do %>
       <%= if Path.extname(config.cover) == ".png" do %>