I'd say that you'd want a trigger that watches your OneDrive, first of all. Probably the "when a file is modified" and point it at the specific folder. I don't think it counts as 'created' since it'll already be created, and you're just moving it in.
When it triggers, get the first 5 characters - as that should be your project #, right?
Get file content for that particular file.
Get Emails - Top: 1, Subject Filter: your project #.
Reply to Email - Message ID will come from your "Get Emails". It'll turn into an Apply to Each, but it'll only be 1 since you selected Top 1. Add your File Name and File Content into those spots, from your "Get File Content" from earlier.