There’s a few things that require configuration for document preview to work. Firstly the file in question must be converted into a PDF file by Liberty Create, before it can be previewed. This can be controller in the Object settings for the File Object (See the Settings tab on the object)
From here you can tick and untick the filetype you want to allow for Preview. Tick all the desired file format you wish to allow to be converted into PDF format and allow for preview on the document preview widget.
Secondly, is checking the preview file and whether it has generated. On an information widget and/or listing widget where your files are placed, use the attribute Preview status to find out if the file is correctly generated into PDF format.
If it says Absent then the file has not generated successfully.
If it says Available then the file has not generated successfully and can be previewed.
It is worth noting that any file formats not listed in the file object, are not supported for document preview.
I hope this helps answer your question and assists you in setting up the document preview. Please do feel free to post if this doesn’t work and i’ll be happy to support you further in getting this working for you