衣服 and 衣饰 both mean "clothing"; but are they truly synonymous or is the use of each term confined to its own specific context?
Or is it simply a difference between colloquial and formal speech or between the spoken the written language?
The former refers to clothes in general, while the latter refers to clothing ornaments/decorations (such as embroidery, patterns, etc).
Krazer has provided a good answer.
You could think of 饰 in the way it comes in 饰品, which means "jewelry" or "accessories". In that sense, 衣饰 is ornaments of textile, and 衣服 just clothes.