PDF files (and, probably, some other mimetypes) aren't listed in thumbnail view

[tsujan]

Although PDF is supported, it doesn't show up in the thumbnail view. That creates problems like the following one:

Steps to Reproduce:

1. Put a PDF file that has image (most do) inside a folder that contains PNG/JPEG images.
2. Drag and drop the PDF into lximage-qt. It'll be shown.
3. Try to change the image by pressing the relevant toolbar buttons.

The image couldn't be changed.

System Information

Latest git LXQt.