filepicker field: JPG extension images are ignored

Hi,

In my blueprint I setup my ```filepicker``` field like this:

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

But the field doesn't show *.JPG files. Should the field include JPG and PNG images in the list by default? "JPG" or "jpg" is just the way we type and save the files, they are all the same file type.

I have Grav v1.3.10 - Admin v1.6.7 .

Reproduce the issue using the supplied filepicker blueprint with uppercase .JPG files alongside .jpg and .png files, then trace the filepicker field's filtering behavior. Done means the field lists both uppercase and lowercase JPG images as well as PNG images; the issue names no implementation file or test to run.

Written by the indexing model from the issue text.