Allow Thumbnail generation for other filetypes such as ODT

[benjaminpick]

#31050 introduced the possibility to add previews to PDF files. This is great, but why stop there? There are many files that really are a ZIP file including a preview thumbnail (e.g. ODT files), or that allow easy generation of a thumbnail.

In a first step, WordPress should enable plugin devs the possibility to add those thumbnail generators over the same path as PDF does (Image Editors). Later, it would be possible to add common filetypes such as ODT to core.

[benjaminpick]

tentative patch