In SS3, I have a CustomImage object extending Image to make use of some custom Image GD functions.
However the upload field for this does not show existing files in the Assets folder. Replacing CustomImage with Image, these files then show again. Any ideas?
public static $has_one = array(
'Thumbnail' => 'CustomImage',
$ThumbUpload = new UploadField('Thumbnail', 'Thumbnail image');
$fields->addFieldToTab('Root.Main', $ThumbUpload, 'Content');
Sorry for the late reply, You are correct: the UploadField will only show files that are part of your CustomImage class. That's because what you see in your UploadForm file list, are not purely images as such, they are dataObjects of a given class, that have an Image attached to them. And you cannot really mix them up...
So the advice is to use a DataExtension for the Image class instead. That way all your 'images' will still be Image DataObjects, but can have extra data attached to them.
See the UploadForm documentation: http://doc.silverstripe.org/framework/en/reference/uploadfield