We just got this email too. I think in response to the myCIC technical issue we filed a while ago.
"Please upload this additional document and attach it to my application" - is there something magic about including this string? If so, why couldn't the call centre guy have told me this three weeks ago? So tonight I'm going to submit the IRCC webform for the 3rd and 4th time (using "technical difficulties" and "sponsorship" categories, since it's still ambiguous which one is correct).
And the start of the email said: "Since the request for additional documents was sent prior to you linking the application on March 21, 2017, in order to upload your documents, we invite you to:". Why does everyone at IRCC keep using this language - "SENT"? Nothing was sent!