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Unfortunately, the OneDrive app for iPhone (iOS) does not include any setting to always show filename extensions. This is not a bug, it's a missing feature on iOS.
The recommended workaround is use the Files app.
- Open Files
- Tap Browse
- Under Locations, tap OneDrive
- You will see the full filenames
To recap what is on iOS OneDrive (Ver 16.26.3)
- No “Show file extensions” toggle
No “List view with full filenames” option
No “Always show full filenames” accessibility option
No setting in Microsoft 365 admin center affecting this
Only Android and Windows versions offer that.