Pimp.my.Gmail series!
If you prefer a script, check out : http://userscripts.org/scripts/show/75047

Tango attachment icons for Gmail! As of now it supports the following extensions:

Archives(rar,zip,7z)
Audio(mp3,wav,wma,m4a)
Images(jpg,jpeg,bmp,png)
Videos(avi,mpg,mpeg,mov,mp4,divx,mkv,flv)
Text(txt)
Document(doc)
Spreadsheet(xls,xlsx)
Presentation(ppt,pps)
PDF(pdf)
Installers(exe,msi)
HMTL(html,htm)
Encrypted mail(p7m,p7s)

I'll be adding more as and when i get time. If you have icons for extensions i've missed out, pls lemme know.
This not only styles the attachment icons in inbox, but also in each e-mails.

Find the rest of the Pimp.my.Gmail series @ "kilroy_@userstyles"
Also check out some of other Gmail scripts @ http://userscripts.org/scripts

CHANGELOG:
v0.1 : 14-09-2010
- Initial release.