darken the attachment-icon

#### before
![image](https://f.cloud.github.com/assets/2665886/1369477/86b54b0e-39f1-11e3-9cd7-05bfdf836960.png)


#### after
![image](https://f.cloud.github.com/assets/2665886/1369479/9b17bbfe-39f1-11e3-91e6-cae5cb9c3d03.png)