[web] Fix "invariant violation" for encrypted uploads

Summary: Fixes ENG-3947. Details are in the Linear issue.

Test Plan: Sent encrypted message on web before it fully uploaded to server. Ensured there was no error in dev tools console.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D8026