Hi
Go to Options - Uploads tab - 'Other allowed File Extensions' add the extension types csv and CSV.
For the Frontend select the User Group that will be allowed to create and edit Downloads and add in ,csv in option the 'Allowed File Types'
If you are unsure about User Groups seehttps://www.jdownloads.net/documentation-for-v3-9/access-user-group-permissions-v3-9/user-groups-settings-overview-2
(remember to set the Ranking)
Colin