How to very easily split filename and append date/time?

Let's say I have:

attachment.FileName = "hello.pdf"

I would like for it to result in:

hello_July5.pdf

I know how to get the date what I am appending, but how do I easily make a split between the "." and then add the variable date between it?

I know this is simple, I am just looking for the quickest fix possible.