Register to open at least some file types

[glemco]

This issue is related to #203 , I get that it's hard to register the app to open all supported file types, so the solution is to register it for literally anything.

In my case, I would like not to have this app pop out to open everything but I'd still like to be able to open some common files (mainly PDFs which this app supports greatly).

If I disable the setting advised in the open issue, the app isn't listed among the PDF viewers and the only way to open files is to manually do it from the app, which is annoying, for instance, for documents received via some messaging apps.

I think it would be ideal to have a middle ground, like open by default the most common files (pdf, docx, pptx, odt, etc.) although the app supports perhaps hundreds of file types, I'd say most people won't use more than 10 of them.