Omni Group’s Omniplan product (Superb project planning tool, by the way), creates a “package” type file that cannot be attached to e-mail. This is confusing for users who see the one “file”, even though on a Mac it is stored as several files in a folder.
Subject: Known issue in Notes 8.5
This is a known limitation in 8.5 that has to do with how bundle files are constructed. We are currently investigating a solution for this issue and targeting a future release. As a work around you can add the file to an archive (.zip) or disk image and then attach.