Microsoft has changed the default behaviour of its Microsoft 365 Copilot mobile app so that tapping a local document attachment no longer simply opens it. Instead, the app uploads the file to OneDrive and routes it through Copilot for AI analysis before you can view the content. The shift, which began rolling out over the weekend to Android and iOS users, affects anyone who has set the app as their default Word, Excel, PowerPoint, or PDF viewer.

What exactly has changed

Up until this latest update, the Microsoft 365 Copilot app—the successor to the old Office Hub and later the plain “Microsoft 365” mobile app—functioned as a straightforward document viewer. You could open a .docx received in WhatsApp, for example, and immediately read it on your screen, ignoring any Copilot prompts.

Now, when you open an attachment, you are greeted not by the document but by a Copilot chat screen. A banner reads: “Opening documents from your device now starts with Copilot chat.” The file is then uploaded to your OneDrive account, and Copilot attempts to summarise it or answer questions about it. If the upload fails or Copilot cannot parse the document—as Windows Latest demonstrated in its hands-on testing—you may be left unable to read the file at all. The classic document viewer has been buried behind the Copilot chat’s references tab, accessible only if Copilot has successfully processed the file in the first place.

The previous experience, where files opened in a local, offline-friendly viewer, is gone for anyone who keeps Microsoft 365 Copilot set as the default handler. Files that are already in OneDrive can still be opened directly via the app’s search page, but purely local files now take this AI-first detour.

What this means for you

Everyday users lose the simplicity of a quick document check. If Copilot fails to analyse the file—as happened in one of the Windows Latest test cases—you cannot simply view the document in the app. This is a regression for any mobile workflow that relies on speedy, offline access to attachments. More critically, every local file you open is silently uploaded to the cloud. A contract, a medical form, a financial statement—anything you receive and tap—may be sent to OneDrive without an explicit consent step. The privacy implications are immediate: your document leaves your device and becomes subject to Microsoft’s Copilot retention policies.

Power users and IT professionals face a data governance headache. The automatic upload bypasses any local-only workflow and may violate DLP (data loss prevention) rules if, for example, a user receives a sensitive attachment via a personal messaging app on a corporate device. Because the upload is opaque, it can be difficult to audit. Microsoft documents that files uploaded to Copilot are retained for a period (consumer docs reference 18 months) and are not used for model training, but for regulated environments—healthcare, finance, legal—the mere fact of unapproved cloud storage can constitute a compliance event. Furthermore, the inability to simply view a file without AI processing can disrupt field workers who need to consult documents in low-connectivity areas.

How we got here

The app’s transformation has been underway for over a year:

  • January–Early 2025: The old Office Hub was rebranded to “Microsoft 365” and then to “Microsoft 365 Copilot,” signalling a shift from a document viewer to an AI hub.
  • October 2025: Microsoft pushed the Copilot chat screen to the forefront, burying the document viewer behind extra taps. The company stated that editing functions would move to standalone Word, Excel, and PowerPoint apps, while the Copilot app would focus on previews and AI-driven summaries.
  • Early February 2026: Message Center bulletins and support documents confirmed that files opened in Copilot Chat (the web version, at least) would be previewed inside Copilot. The mobile app update that removed the local viewer appears to be the culmination of that AI-first strategy.

Microsoft’s official rationale is that starting every document open with Copilot provides “quick summaries of important information” and lets you view the file anytime from the chat. The reality, as user reports show, is that the feature often fails to deliver a readable summary and leaves the user stranded between an unhelpful AI response and an inaccessible document.

What you can do right now

For everyday users

  • Stop using Microsoft 365 Copilot as the default viewer. On Android, clear the defaults for the app in Settings > Apps; on iOS, use the “Open in” menu to choose Word, Excel, or PowerPoint when tapping an attachment. These standalone apps still open files locally.
  • Use the OneDrive app if you intentionally want a file in the cloud, but manually upload it yourself rather than relying on Copilot’s automatic behaviour.
  • Uninstall or sign out of the Microsoft 365 Copilot app if you never use its AI features. Without an active account, it cannot act as a default handler.
  • Monitor OneDrive’s recent uploads if you suspect files have been sent without your knowledge. Check the OneDrive app or web interface for any unexpected documents.

For IT administrators

  • Conduct a pilot test: Install the latest Microsoft 365 Copilot mobile app on a managed device and document exactly what happens when opening local attachments. Log OneDrive activity and any Copilot interactions.
  • Review DLP and Conditional Access policies: Verify whether your rules catch or block uploads initiated by the Copilot app. You may need to add the app’s client ID to restricted lists or adjust policies that allow uncontrolled cloud uploads.
  • Manage the app with MDM: Use Microsoft Intune or another MDM solution to block installation of the Copilot app, restrict its ability to be set as a default handler, or silently prevent it from running on corporate devices.
  • Update end-user guidance: Clearly communicate that the Copilot mobile app no longer behaves as a simple viewer and that sensitive files should be opened only in the dedicated Office apps. Provide step-by-step instructions for changing default apps.
  • Seek tenant-specific clarifications from Microsoft: If your organisation has strict data residency or retention requirements, raise a support case to ask exactly how the Copilot mobile upload flow works, where files are stored, and what audit logs are available.

Outlook

Microsoft’s pivot to an AI-first mobile experience is not surprising, but the execution here has created a trust gap. When an app that millions of people use to simply read documents suddenly starts uploading them to the cloud without a clear, in-the-moment permission screen, the reaction will be negative. Expect Microsoft to address this—perhaps with a more explicit consent dialog or an admin toggle to disable automatic uploads—in a future update. In the meantime, treat Microsoft 365 Copilot as an AI gateway, not a document reader. For now, the safest path is to keep your local files local by using the standalone Office apps.