Last week I testet the option to attach a powerpoint file when creating a Skype For Business meeting. As I remeber I was able to present this powerpoint file in the Meeting. Now when we are testing the I can only press manage content and see the Powerpoint file, but I´m not able to present it.
When looking at the manage content window, do you see it listed under Attachments or under "Prestentable Content"? If under the latter, verify your Office WebApp server is available (presuming you are on SfB on-prem).