filepicker produces error with multiple and preview_images

[bitstarr]

There is a wierd situation which produces an error on load of the admin edit view:

• a filepicker field with preview_images: true and multiple: true
• with one file selected

Bonus: The preview image gets shown with one file selected, but not with multiple files selected.

I also can confirm it's not caused by number-only filenames (3.mpg) or files that are not images. This keeps reproducing with all-images content too.

I investigated this after it was mentioned at discord but i'm not smart enough to get my head around what's exactly happening in themes/grav/app/forms/fields/filepicker.js.

Here is an example blueprint snippet:

            header.gallery:
              type: filepicker
              label: Gallery
              folder: 'self@'
              preview_images: true
              multiple: true
              validate:
                type: commalist
              accept:
                - jpg
                - jpeg
                - png