Gmail always displays a paper clip icon next to the date of an e-mail indicating a file is attached to that particular e-mail by normally. I developed a Chrome extension named Attachment Icons for Gmail™ and Google Apps™, it's available on Chrome Web Store now. Once it's installed on your Chrome browser, the paperclip icon is changed to a thumbnail representing the file type attached.
This extension adds different icons for different email attachments in Gmail and Google Apps mail, a DOC icon for emails with Word documents, a PDF icon for emails with .pdf files, and so on. With this extension, you'll be able to quickly identify if it's a JPG, PDF, MP4, PPT, or any other common file type attached to your e-mail without even having to open it.
This extension supports multi-language, it can be easily localized. If you like to help make it available to more users by translating it into your favorite language, click here.
If you have any suggestions about this extensions, please leave your comments here. :)