Your company may have chosen to disable your ability to send files.
Click the plus button at the bottom of a chat window and click File:
Find the file you wish to send and click Open.
A representation of the file, including its name and size, will now appear in the edit control at the bottom of your chat window. Confirm it's what you wanted to send and press enter or click Send to send the file:
You can also drag and drop files directly into a chat window.