There is no easy way to do this at the moment. You could technically create an activity backup, rename the MBZ extension to ZIP, unzip the backup and fetch the attachments from the files directory in the archive. However, the attachments in MBZ will have a name generated by Moodle. The original file name (and the owner of the file etc) is stored in XML
files in the archive.
Please feel free to create an issue (of a type Improvement, component Workshop) for this feature request in the tracker. Let us see if other community members would like to see this implemented as well (measured by collected votes). The number of votes helps our decision makers to set up priorities for the further development. Thanks in advance.