If you want to restrict file upload by file type/extension/format you can follow the below steps.
Goto File Browser Item --> Advanced --> Custom Attributes
Then pest below code :
accept=".pdf,.csv,.xlsx"
Here it means only pdf , csv and xlsx file will be acceptable for upload.
If you want to accept only pdf then you have to put like below code as the screenshot given:
accept=".pdf"
Double-click on the image to view it on a big screen.
That's it. Now you can go and check.
