LPS-39361 import previews

Hi @KocsisDaniel,

I guess the approach followed by @peterborkuti was the best for me. Using getPreviewFilePath and getThumbnailFilePath will require an API change and probably that wouldn't be the best for patches and the inminent 6.2 release.

I followed the same pattern as in Video/AudioProcessImpl, so it's ok, and it works.

The other possible way is changing DLPreviewabeProcessor to be like this (in this case better to iterate over getPreviewTypes()), and change PDFProcessorImpl to have the same behaviour as DLPreviewableProcessor has right now, so we can avoid implementing deletePreview() in ImageProcessorImpl, VideoProcessorImpl and AudioProcessorImpl.

@caorongjin Alex perhaps you want to take a look at this.

Thanks Daniel!