Despite saying otherwise, it (in my case at least) does not actually support docx files. IT gives me an error every time I do. Furthermore, it won't let me change my resume either. It acknowledges that I tried, as evidenced by a change in the timestamp of last upload, but every time I check it out, it is always showing my very first one uploaded, even though I have changed it since then. The autofill is nice, it just would be even nicer if I didn't have to manually reupload my resume to every application because Simplify won't let me change it.

Edit 8/30/2022: they added a resume viewer since I first wrote this so that we can compare what they have stored to what we actually have. It helps, credit where credit is due, but it is still not letting me upload docx files despite being told that those are allowed. The support team is nice and willing to help, which is nice to see.