Hello Prashant,
Thank you for reaching out to the Microsoft Community and I'll be glad to assist you today.
This indicates that the website only supports specific file types for uploading, so other file formats aren’t allowed.
When the mini-file explorer window opens on the browser, click the arrow next to "All files" to see if your files are displayed as expected.
• I also recommend trying a different browser, such as Edge or Chrome, to see if you can upload the files from your file explorer.
• If the issue persists, try moving the file or folder to a different location or drive and check if it becomes visible for uploading.
I hope this information helps. If you have any questions, please let me know and I will be happy to help you further.
Best regards,
Gunasekar N