Desktop
You can grab a link to a previously uploaded file without leaving the channel you're chatting in. Just click Choose file 
beneath your compose box, then select Browse Teams and Channels. Go to the file you want, and then click Share a link. Teams posts your link in the channel conversation, including the name of the team and channel where the file is stored.
Alternatively, you can go to the file's location in Teams, click More options 
next to the file name, and then select Get link. You can now paste the link into any channel or chat.
Don't forget that you can share links to files in other channels (even if they belong to different teams). That way, you never have to upload multiple copies of the same file to work with it in different places.
Mobile
In the files tab of a chat or channel, tap More options > Copy link to get a share link. Then, you can either send in a message, or share in the app of your choice.