TL;DR: Do provide extra info when possible. But: Focus on providing self-contained data in your post; don't rely on external uploads entirely.
There's a rule that is more strictly enforced on Stack Overflow, especially for some tags: every question should contain a minimum working example. A Short, Self Contained, Correct (Compilable), Example, to be precise. Now, I guess that on Super User, there are some cases in which the question is fairly easy to understand and doesn't need that, but let's assume we do need some data for understanding the problem and its context. This is often the case for spreadsheet questions, scripts, etc.
How do you provide this data?
The simplest way to provide this kind of example is—like you say—adding a screenshot and a minimal data table. Maybe include some formulas as well, if they're required to understand your issue.
What we're really trying to achieve is making the Stack Exchange post self-contained. If that's impossible, perhaps you need to research a little more and get to the root of the issue first.
Should you provide supplemental files?
If you have them, yes, you should! If it helps, there's no reason not to share this kind of material. Supplemental files could be spreadsheets, video files that don't play, some huge logs (well, there's PasteBin for that, but you get the idea), et cetera. You can use popular services like OneDrive, Google Drive, and Dropbox to upload such files.
However, the question shouldn't be:
Here's an Excel file:
<link> How do I do XYZ?
I have an Excel file containing data ABC. There is a formula that calculates DEF from columns 1, 2, and 3. How do I do XYZ?
What if the files go offline?
External links are very prone to rot. We can't change that. You should therefore:
- make an effort to have your files be accessible for as long as possible, not just temporarily
- write the question in such a way that it is understandable without that supplemental material, in case the link ever goes down
What about documents containing malware?
Is this really an issue? I think the benefits of adding files outweigh the risks of documents containing malware. Obvious spam or malicious posts are quickly identified as such anyway. And in the (very very few) cases I've seen where some documents were suspicious, the community was very quick to react to it, at least removing links or prepending them with a note.