This will explain how to make a post in Teams, and link to a file that has already been uploaded to the Files of one of the Team's channels. Note: at this time, Teams does not do well with playing large, shared video files within its software. For this, we recommend Microsoft Stream.
First, choose the channel in which you want to post:
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Click in this box at the bottom of the Teams window (remember that you can hit TAB + ENTER while your cursor is in the box to have more formatting options and also disable the Hit Enter to Send feature):
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After you get done inputting your text, click on the paperclip icon:
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Take this option on the popup:
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This window will come up, and will show no files if there haven't been any files added to this channel. Click the up arrow at the top left of the window to navigate to files that exist somewhere else in the Team:
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After you're found the file, click on it, then choose Share a link: 
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Click to post:
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The published post with its link:
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