Attaching OneDrive File to Email in Outlook App I'm using the Outlook mobile app for iOS as well as OneDrive. I'd like to attach files from OneDrive when I compose an email in Outlook, so I click on the paper clip, then choose Attach File, then there is a page titled "Files" and at the top are a list of cloud service providers, such as Google Drive, Box, etc. (picture at the end)

Unfortunately there is an X next to each of these, and I accidentally clicked on the X by OneDrive, assuming it was an arrow ">". Now that option has disappeared. Have I permanently lost the ability to attach files from OneDrive to my emails? How do I bring this back?

It's not clear to me if this "Files" page is just a part of Outlook, or if it's related to the Files app bundled with iOS. The whole thing is a mess.
This modal is unrelated to iOS Files app — it’s a custom presentation by the Outlook app. The app displays common account providers in the list to help promote the ability to log in to these providers for the purpose of attaching files, with the ‘x’ button to dismiss this.

The ‘normal’ way of adding a storage provider to Outlook with or without actually having a file to add is through Outlook’s settings. Open Outlook’s settings using the gear icon from within the app, then choose Add Account followed by Add Storage Account. You can then choose a provider from the full list of available providers.