If you have the mouse buttons switched in the windows settings (for left handed users) and switch the mouse buttons in the game to work as expected (as the game overrides the windows settings)
then the game will not let you select weapon attachments with the mouse. If you click on them, then you fire your gun, instead of selecting the attachment.

Tested on:
Win 7 x64 Pro
Nvidia 9800GT EE
AMD Athlon 64 X2 6000+ 3.11GHz
4GB RAM
and Razer Krait mouse.
All drivers are up to date.

You can edit these settings in-game via the options menu.

I can easily remap the buttons so my primary click is fire and secondary is sights, but the problem is that then the engine does not recognize a non left click "fire primary" as a selection method in the weapon customization screen. Instead of selecting different weapon attachments, it closes the weapon customization screen and fires the gun.

to view the bug just set "fire" to "mouse 2", and "sights" to "mouse 1" then, in game, access the weapon customization screen, and then try to change the attachments. "Mouse 2" (which should now select) will close the menu, while "mouse 1" will do nothing (as expected).