Technically, in terms of meaning, both are correct. "As" can be used to show causation, "because" definitely can, and while we're at it, "since" can be used as well:
Since we need that function, we must import the following properties file.
For some reason, though, the first form (the one that uses "as") sounds less natural to the ear. It may be because I'm used to seeing "as" used to convey a sense of simultaneity, e.g.
As we were walking to the market, we came across a giant well.
Of course, this conjunction has many uses, as the dictionary definition on Free Online Dictionary shows. It definitely can be used to show causation; it's just a matter of personal preference.