Uploads are not queued for sending later on connectivity loss

### Description

Unlike messages which get queued for sinding later when the internet connection disappears intermittently file uploads just fail with an error message. The should be put into a sending queue like normal messages and retried when the server connection is back.

### Steps to reproduce

- Upload a file to a room
- lose connection to your homeserver
- get popup message that the upload failed.
### Version information

riot-desktop from Archlinux repo, riot-web version: 1.6.0