If you need to create a new file in Word, PowerPoint, or Excel, you can add one right in Yammer.
Create a file
From your Yammer home page:
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Select Create a File.
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Select the drop-down arrow and select the file type you want.
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Type in a file name, and select Create.
The new file opens in another tab, and also appears in a Files list that all group members can access.
Edit a file
From the Files list in your Yammer group:
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Select the file you'd like to modify.
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Select Edit File.
Source: Microsoft