If the file you're about to attach to your internet email can be found elsewhere on the net, it maybe better to send the address of that location rather than attaching a copy of the file itself. If nothing else, this will save you some time because your system doesn't have to transfer the large attachment. Also, your recipient(s) can decide whether to download the file or not, where as if you had attached it, they may not have any option but to take the time to download it even if they don't want to.
On the other hand, if you're sending intranet email, it maybe better to attach the file. This is because, unlike with internet email, most intranet email does not have to travel too far on slower links. Also, going out to the internet [from your intranet] to get a file usually takes longer than getting it from a local intranet email.