Drop files into conversation cannot be uploaded

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Steps to reproduce

1. Drop a file into a conversation
2. The file appers in my conversesion but failes to upload

It works when I use the + sign to upload a file.

Expected behaviour

The dragged file should be added to the conversation as when I click the "+" button to attach a file.

Actual behaviour

The dragged file cannot be uploaded

Diagnosis and logs

{
  "launchAtStartup": false,
  "theme": "default",
  "systemTitleBar": false,
  "monochromeTrayIcon": false,
  "zoomFactor": 1,
  "playSoundChat": "respect-dnd",
  "playSoundCall": "respect-dnd",
  "enableCallbox": "respect-dnd",
  "secondarySpeaker": false,
  "secondarySpeakerDevice": null,
  "lastAppVersion": "2.2.0"
}

Start by reproducing the failure in the conversation drag-and-drop upload flow and compare it with the working "+" attachment flow. Trace the desktop conversation UI entry point and the upload handling used by both paths. Done means a file dropped into a conversation uploads and is added just as it is when selected with the "+" button.