To attach a file or picture in a channel, click Choose file beneath your compose box. Then you can upload an file or picture from your computer or OneDrive, or search for one from the Files tab for that channel.
In a chat, click Choose file and pick the file or picture you want from your computer or OneDrive.
Send a picture
To attach a picture to a message, tap Add photo beneath your compose box to take a photo or to open your photo library.
Send a file
To attach a file to a message, tap Choose file beneath the compose box and go to the file location.
From a chat or meeting, tap the attachment to open it with an Office 365 app. In a channel conversation, tap the attachment or go to Tabs > Files.