Hi radmir,

Please check your handler file. We using "UploadHandler.ashx" in the demo, and has the AllowedFileExtensions setting there. did you just copied the demo handler file and use on your own site so did not remove the AllowedFileExtensions setting?

option.AllowedFileExtensions = "*.jpg,*.gif,*.png,*.bmp,*.zip,*.rar,*.txt,*.exe,*.doc,*.docx,*.pdf,*.wmv"; 

Regards,

Ken