filepicker is not working with sub-directory multi-site.

I am trying to use the filepicker of a blueprint in a subdirectory of a multi-site.
However, even if ' self @ ' is specified, the image list is not displayed at all.
Filepicker I wrote in the main directory blueprint, the list of images came out.

Also, if you type directly into the default.md header, the file name will be displayed, but the thumbnail will be Not found.

Grav: v1.6.6
Admin: v1.9.2

```
header.image:
type: filepicker
label: TestImages
folder: 'self@'
preview_images: true
accept:
- .jpg
- .png
```

Could you help me to solve the problem?

thanks.