Attach a File in a Message Post
Open Microsoft Teams and select a team to send a file to.
In a channel, under a message box, select Attach files. Select how you want to attach it:
Recent: Attach recent files.
Browse Teams and Channels: Attach files from a team or channel.
OneDrive: Select single or multiple files from a OneDrive for Business account.
Upload from my computer: Upload files from your device.
Select a file > Open.
If you'd like, add a message and @mention someone in your post.
Upload a file to the Files tab
Every channel has a Files tab. You'll find everything that's been shared there.
Select Upload > Files or Folder.
Choose your files.