Normally Gmail uses a paperclip icon to let you know a file is attached to the email. The free Chrome extension Attachment Icons for Gmail replaces the paperclip with standard Windows file type icons for PDFs, pictures, spreadsheets and many others.
The extension does have a bug or two — if the email has multiple types of attachments only the first icon is displayed and if you use Gmail themes the icons may not display properly. Other than those two nitpicks the extension works well. I'm adding it into my setup.