I have developed and updated a medical application for cardiac ultrasound since 2011. One feature of the app is a user-generated document library for documents, images, and movies, all displayed using QuickLook.framework. QuickLook has functioned well until now with iOS 10. At this point .csv files (generated by the app to store calculated data) display as simple text (including the commas) instead of as a spreadsheet. I have not found any Apple documentation to explain this change in behavior. The log in Xcode shows no error messages when displaying a .csv file.
I am using the iOS Simulator at this point to test my app with iOS 10. However, I have verified that the El Capitan (10.11.5) version of QuickLook opens a .csv file in the simulator library on the desktop without a problem, with the expected spreadsheet format. Therefore, I do not suspect any problem with the .csv files that my app is producing.
I suspect this problem with .csv display is a bug in iOS 10, but with no resolution after 3 beta releases, I am concerned that the problem persists. Any help would be much-appreciated. My hope is that there will not be a need to adopt a new document display method, since QuickLook has worked so well for several years now.
Of note, my app does not use any Swift code.
iOS 10 is still in beta and will be in beta until at least the second half of September.
One of the main advices there is while testing beta software is to test it on the real device with something like iOS 10 Public Beta build installed.
If you are 100% positive that this is a consistent, well repeatable bug on Apple's side (not on your own or any other 3rd-party library you might use) consider filing a radar.
Concerning your specific problem, as far to my knowledge, Apple didn't make any significant changes to the QuickLook functionality in iOS 10, but once again, you can prove me wrong, by carefully browsing through all the latest changes in the official iOS 10 changelog.