TB should offer three menu options for each file:
(a) Open as tab (aka Preview)
(b) Open in external app
(c) Save As...
with one of them being set in Settings as the default by the users. It's a very simple enhancement really, and an established way of doing things.
If TB can't do (a) for certain file types, it should only offer (b) and (c).
At the moment we are forced to choose one of these or 'Always Ask' per file type in Settings and our choice applies to all files of that type. If I must choose, I have to go with opening all files (eg PDFs) in my external app and miss out on TB's quick preview feature which I would use in most cases. Alternatively, I can save the PDF and open it from the file manager, but that's too many unnecessary clicks.