Include a html-file in a post

If I want to include a file in a single post, just a simple html-file with simple code in it, how do I do that?

I’m using like three different ads in my posts, and if I have to change an ad, then I have to edit each and every post that has that ad in it.
It would be much easier if I just had to change one single file, and then all posts using that specific include file would be changed.

Haven’t been able to get it to work.

Any ideas?