To add attachments to mass event forms like the "Attachments" section, available in the usual event form, please do the following:
Go to Start Event-> Click ‘Select your assets now’ (under Multiple Assets)
Search for assets-> Select ‘Perform an Event’-> Select an event type-> Click Continue
This will open the Perform Mass Event screen.
At the bottom of the page, below Event Schedules, add an Attachments section:
Upload multiple files by selecting ‘Choose Files’
Remove the attachments by clicking on the remove link
Add comments to the attachments
Click "Save," the uploaded attachments should get saved to all the events selected in the ‘Mass Events'
Note: If the user clicks Cancel, the event would be cancelled