You just sent a file or made an upload request, and you see in your sent items from Outlook that no button is visible. This is often due to the fact that the e-mail was sent out without formatting. In that case, Outlook sends the e-mail without HTML formatting to the recipient. He also extracts the layout and images. The text is therefore a bit sloppy looking.
Fortunately, this is easy to solve! When sending an e-mail you can choose 'HTML' or 'Text with layout'. In both cases the messages are sent correctly and the button will be visible again. You will still have to send the message again (in the right way), so that the recipient also sees the button.