Afghan insurgents stage a raid dressed in US Army uniforms. So, how did they get the uniforms?
The Army hires civilian contractors (mercenaries) to transport clothing into the country; the mercs sell uniforms complete with body armor to the Taliban. Soldiers hire native servants (Dhobi and darzi using the terms from the British Raj) to wash and mend their clothes, some of which will go missing. And I suspect some soldiers will sell the uniforms of dead colleagues for fine Afghan opium.
There are many ways for an Afghan rebel to find American uniforms and American weapons.