how to restrict a document library to upload only specific file format in sharepoint

just go to sharepoint centeral admin

------> then click on security it will display option like below

by clicking define blocked file types  u will goes to below screen

just add the format like i did docx format if i upload the document like docx it wont allow u to upload that document it will show below screen

if any one knows how to do same thing in office 365 online let me know