This has been a long awaited capability and painful user experience.
As you know, it has been impossible to share files in external chats as shared files are saved in user’s OneDrive.
Well, starting early July (targeted release tenant – aka preview) and mid July, you will now be able to share files in external chats.
Note, this is the expected schedule; it may vary a bit.
To allow this, Teams administrators will have to configure the FileSharingInChatswithExternalUsers setting on files policy.
Connect-MicrosoftTeams
Set-CsTeamsFilesPolicy –Identity <policy name> -FileSharingInChatswithExternalUsers Enabled
You can check the configuration of the file sharing in chats using the command
Get-CsTeamsFilesPolicy