OXT files (application/vnd.openofficeorg.extension) are extension files for Libreoffice and so shouldn't looks like rich-text:

A proposal icon:

I used unfolded corner because it's not a readable file but shouldn't use it to keep consistency with other LO mimetype which are folded now? @ubuntujaggers ?
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A proposal icon:
I used unfolded corner because it's not a readable file but shouldn't use it to keep consistency with other LO mimetype which are folded now? @ubuntujaggers ?