We create thumbnails for videos (via `ffmpeg`) and upload them along with the actual video to the `keyserver` for video messages.
We want to unlink the temporary file once we're done with it so we don't clutter the user's temporary file directory (eg `NSTemporaryDirectory` on iOS). We were previously unlinking the temporary file once the video was successfully uploaded, but we should first await `waitForCaptureURIUnload(...)` to ensure that the image isn't being actively displayed in the UI when we unlink it.
This matches the behavior of captured images and pasted images (we create a temporary copy because we can't count on clipboard contents remaining the same) below (~line 813-836).