If the file you want to share is less than 500 KB, you can put it as .zip file and upload it to the forum as attachment (the same way you upload a single pic to a post).
If it's more than 500 KB, then you'd have to upload it to a 3rd party hosting site (i.e. another website)...
There are a variety of such sites, each with advantages and disadvantages, may I recommend...
http://www.googlepages.com/http://www.4shared.com/Take Care