In your channel conversation, click Attach under the box where you type your message.
Select from these options:
Recent
Browse Teams and Channels
OneDrive
Upload from my computer
Select a file > Share a link.
If you're uploading a file from your computer, select a file, click Open, and then Send .
You can always see all the files you post to a channel by going to the Files tab.
For more information: https://support.office.com/en-us/article/chat-and-share-files-in-teams-d7978db0-33b5-4ad3-93ac-ef0bd3c2a670?ui=en-US&rs=en-US&ad=US