Add extra format support (.doc, .xls, ...)

[Xandrr]

Thank you for this excellent plugin!
Please add some other formats support (.doc, .docx, .xls, .xlsx, .msg, .rtf, .txt, ..)
I see, some document formats are supported by forks of this plugin, but those forks are too old and don`t works with the last redmine.

[tofi86]

@acosonic requested docx support in #62 mentioning https://github.com/lrusso/TinyDOCXViewer as a preview possibility.

Currently I have no time to develop this enhancement but I'm happy about code contributions! :)

[acosonic]

There is also this library... https://viewerjs.org/ for viewing of odp and pdf's...
Source is located here... https://github.com/kogmbh/ViewerJS

Might be good idea to consider it also...

[acosonic]

@tofi86 hi, check this out:

https://github.com/HugoHasenbein/redmine_preview_office

I have tested it, and I can confirm it works, what it does, basically, it calls headless libreoffice, and converts docx, and other formats to PDF, then performs preview...

soffice --headless --invisible --convert-to pdf FILENAME

So, basically, you can either use that same peace of code, or make that plugin
as optional dependency...