If you are not allowed to upload a specific file, please make sure that the folder is writable, the file is not too big and it has an allowed mime type.

Allowing more mime types

By default there are only a few mime types allowed to be uploaded. Go to site/panel/config/config.php to add additional mime types.

Thumbnails for uploaded images

Kirby will automatically try to generate thumbnails for your uploaded images. If this fails, it will use the original files to display a preview in the file browser. This can influence loading times, when you've uploaded huge images.

To make sure that Kirby is able to generate thumbnails, please add a thumbs folder to the main directory of your Kirby site and make it writable.