"Not Found" when trying to download files

Sir, we face another problem regarding while restricting file upload extension and upload size. As we are using it very first time we don’t know how to do it. please help us.

The information you provided is very scarce. In order for us to be able to help you you need to explain what exactly is happening. Probably you have to configure your web server correctly but not sure.

we try to change STORAGE_BACKENDS for indico. after any upload files are stored there but when we try to access uploaded data via web it is not visible. it is showing this error

Not Found
The requested URL /event/10/abstracts/18/attachments/64/IMG_2810.JPG was not found on this server.

Are you sure the files are correctly “added” when creating/updating an abstract?

Check the log files of your webserver (inside /opt/indico/log/) - a 404 when accessing stored files (assuming the files are correctly stored) usually indicates a misconfiguration regarding xsendfile/xaccelredir.