On the weekend we run into the issue that you can store files in a container, but you can't simply drag them into other applications or to the Finder.
To enable the desired behavior with a new plugin function called FM.AllowFileDragDrop.
The plugin than intercepts the drag and drop in FileMaker and writes the data into a temp file, so the drag includes a file reference and does work with Finder and other applications.
Watch the video: AllowFileDrag
Requires MBS FileMaker Plugin 7.0pr2 plugin or newer.