You can test the blueprint upload on latest version 5.7.1 (209835fc6), published to
Since blueprint upload works using external service, I'm not sure how to make it work from localhost and didn't spend too much time on testing this other than repro using the published app.
See TP entry
It has attachment Test PDF.pdf, used to both discover the issue by BoConcept and repro by VW.
So, I'm not sure if this some kind of limitation of the feature or if it's a bug. I've only tested pdf upload for blueprints when the feature initially was released and it seemed work back then.