File upload issue in non-channel meeting with externals

My law firm regularly meets with externals over Teams. These do not occur in any Group or Channel. They result from creating a new meeting over Teams from Outlook. The issue is that we have been unable to share files with other meeting participants. It does not work to drag-and-drop into the chat field; there's a bar/circle/can't-do icon. Not all, if any, participants are Guest User objects in Entra. (One example would be a deposition, where the witness and court reporter are not Guest Users, but there's a need to share exhibits that cannot be distributed before the deposition.) Is there any way to transfer files to all participants in this scenario?