Thanks for your posting! It’s currently not possible to add attachments to multiple test cases in one step but you would have the following options. If the cases are in same section or otherwise related to each other, you could look into uploading the attachment to one of these cases and including the attachment link in the description of the section, for example. You can alternatively also do the same on the suite / case repository level. Would this maybe be an option for you?
You can alternatively also add the same attachments to multiple case of course but this would currently require attaching the file separately to each case.