Consider DjVu files to be documents

Summary:
BUG: 369195

Based on the Wikipedia page (https://en.wikipedia.org/wiki/DjVu), it seems reasonable to consider DjVu files to be documents rather than images.

Test Plan:
Tested in KDE Neon. Compiled and deployed fine. .djvu files are now listed as Documents:

{F5453411}

Reviewers: vhanda, #frameworks, rkflx

Reviewed By: rkflx

Subscribers: rkflx, #frameworks

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D8528