How do i insert a link to a file into the content?

I am writing my first plug in and would like to know how/if i can reference an external html file. The issue is that a customer has a page that is too specific to be able to cut and past into the content editor. I would therefore like to be able to place a tag in the content that is replaced with a n insert of the file rather than from a function.

I’ve got as far as writing the plug in with a str_replace function to swap the tag with text. I’m just wondering if i can str_replace it with an html (or whatever) file instead.

Any help would be well appreciated.