Windows 10 is known to upload random end user docs to Microsoft. For example, if a computer with Win10 on it crashes / has some level of fault (etc), it can upload docs it determines are related to MS (to assist diagnosis).
To bad if those docs happen to be medical records - or anything else sensitive. :(
Note - that's just one example. Win 10 really is a privacy shit show.
Apparently it's possible to temporarily turn it off.
Saying temporarily because there are (apparently) settings which can be used to minimise it (unsure about "complete off"). The setting names and meaning appear to be unstable though. eg the required settings change name, change their meaning, (etc) with subsequent Win 10 updates and releases
Just in case that sounds like it might be "on purpose" by Microsoft in order to make things difficult for their targets/end users to stop this abhorrent behaviour... it does seem that way.
Microsoft is absolutely not operating in good faith. :(
For in depth details, there's plenty of info available by looking online. Using HN's search feature should bring up a wealth of helpful pointers too.
Personally, I can't tell you the settings needed for disabling this stuff, as I've never run Win10 (due to this crap).
Everything possible has been migrated to non-Windows, and the last few pieces (running Win7) are on not-connected-to-the-internet VM's or hardware while non-windows alternatives are being tried out.
Yes, it's determined by whether the diagnostic data setting is at Full or at Basic. This is presented as a toggle in the initial OS install setup dialog and can be changed later in Settings->Privacy->Feedback & diagnostics. The only change to this since the initial version of Windows 10 (besides reducing the data collected at Basic level to comply with European law and regulations) was that the third, intermediate Enhanced level can no longer be set through the UI (it still exists as an enterprise configuration option) since the Creators Update release in spring 2017.
Yes, it prevents the scenario (user content files being included in diagnostic data) that you'd written about, and that the question was about. It doesn't turn off all diagnostic data, but that wasn't the question.
To bad if those docs happen to be medical records - or anything else sensitive. :(
Note - that's just one example. Win 10 really is a privacy shit show.