During a Lync Meeting, file sharing is more subtle than in an Instant Message.
With the conference in progress, PC users should click on the Presentation icon, then the Attachments tab and then click Insert Files.
Mac users should click on the Paperclip icon in the toolbar.
If a Mac user has already received files in the meeting, then they will need to click on the Paperclip icon followed by the + symbol below the list of received files.
Mac users can remove the availability of their attachments by clicking on the Paperclip icon and deleting their shared files from the list.
PC users can click on the Presentation icon, then the Attachments tab and then the Manage Attachments button to perform these actions: