Generating an attachment from field data

Hello, I'd like to generate an attachment based on field data, when the State field becomes a certain value. I already have a script set up to check when State becomes “X” - and in this script, I'm formatting a rich text field the way I want it. Then the user can copy/paste into MS Word. However, it would be better if I could generate a .docx file directly in my script, to better control the formatting. Is there a way to do this? I have read the REST API discussions on the forum and it only seems possible to attach existing files via REST, but in my case I want to create a file attachment “from scratch”. thanks