Please help us help you and fill where relevant:
Your LimeSurvey version: [Version 5.2.14+220214]
Own server or LimeSurvey hosting: The school is hosting LimeSurvey on its servers
Survey theme/template: Vanilla
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Hi,
We have recently upgraded LimeSurvey from 2.5 to 5.2.14+220214 and our users are having trouble with uploading files to the survey as it keeps giving an error (the file extensions and size are observed). Please find attached. I would be most grateful if you could help me resolve the issue.
Thank you.
Regards,
Alex

I recommend to update to the newest release of LS5 first.
I don't think a direct upgrade from 2.X to 5.X is tested widely.
I would also recommend to install a fresh LS5 on the same webserver to see if the issue is caused by the upgrade or by the serversettings. It still can be bug in LS5 only exposed by your setup. The error message is not giving me a direct clue. Are you ruling size and filetype extensions out?

Hi Jelo. Thanks for your response. Will look into reinstalling a fresh LS5. In the meantime, I tried all file extensions and sizes - same error.
With thanks,
Alex