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Please have a look at the app "Marked 2". You can drag the file icon from Textastic's window title bar onto Marked 2's Dock icon to quickly preview Markdown.

Hello, which error messages do you get when you try to upload files?

This should be fixed in Textastic 6.4.2.

This should now work with the latest versions of Textastic (6.4.2) and Working Copy (3.1.7).

I don't have a OneDrive for Business account, but error 403 is "Forbidden". It sounds like it doesn't have the correct credentials (user and password).

For the new iOS 11 Files app, the file provider of Working Copy had to be rewritten. Just last week we found a new way to share information about the source app (Working Copy) with the app that opens the file (Textastic) on iOS 11.

There will be a new version of Working Copy released today and I plan to release a version of Textastic with support for this this week, too. 

I'm working on this. I'll also add the ability to change the sort order of files.