Mobile uploads work, web uploads fail: test one file

Compare a harmless file of the same type and size on mobile and web. Note whether the web browser rejects the selection, stalls during transfer, or finishes but fails to process it. Record browser and app versions, local time with timezone, and any extension or permission that may affect the browser. A private window can be a useful control, but its success is an observation rather than proof of a cause. Do not share the file or its private name merely to report the bug. Identify the exact failing stage.