Attachment generation: preview image

Goal: create a queue that generates default attachments for video medias.

### Example

```
# Preview image
ffmpeg -i input.mp4 -ss 00:00:03 -frames:v 1 -s 1280x720 -y preview_image.webp

# Preview animation
ffmpeg -i input.mp4 -lossless 0 -ss 00:00:00 -t 00:00:03 -s 320x180 -qscale 60 -y preview_animation.webp
```