* src/converse-controlbox.js
async/await causes many tests here to fail due to the controlbox toggle
now apparently showing up a little later (and the tests don't wait for
it to happen).
* src/converse-minimize.js
We get timeout issues in tests.
* src/headless/converse-chatboxes.js
We get a stack overflow while running tests.
With the recent fixes in backbone.browserStorage, the 'file' attributes
gets persisted to browserStorage before Converse tries to upload the
file.
The File object can't actually be serialized, so an empty map gets
stored and then returned together with a 'change' event.
Instead, store the file as a direct attribute on the message object.
Fixes#1261
