We should substitute Rails related paths on files only

Related to #291

Author: akzhan

lib/overrides/premailer/premailer.rb

@@ -9,7 +9,7 @@ def load_css_from_local_file!(path)
     # Remove query string and the path
     clean_path = path.sub(/\?.*$/, '').sub(%r(^https?://[^/]*/), '')
     rails_path = Rails.root.join('public', clean_path)
-    if File.exist?(rails_path)
+    if File.file?(rails_path)
       load_css_from_string(File.read(rails_path))
     elsif (asset = Rails.application.assets.find_asset(clean_path.sub("#{Rails.configuration.assets.prefix}/", '')))
       load_css_from_string(asset.source)