Did some setup for you to clarify problem - yes it is a 404 error but do not think it is jD - never had it before!!
I set ups as below
new user group called UploaderUG with parent Registered
new access called UploaderView with UploaderUG as its only member
new user called Uploader member of UploaderUG
joined users Colin m and Super User to UploaderUG
This is so we can set required User Groups Settings in one place with a ranking of 123 which is higher than any one else
Added two menu items List all Cats - with Registered view access & Create Download with Uploader View Access
Added user uploader
In Category 1 I have given UploaderUG permissions to Create, Edit and Edit Own. It has also of cours inherited Download permission. These permissions are also inherited by the Downloads. When you create another Top level Cat then you will need to allow the same permissions.
Have set default image as the 'Select' so it should find its own extension pic.
You have a 2Mb limit for uploads due to settings in your php.ini file. Link below might helphttp://www.jdownloads.net/documentations/item/how-can-i-upload-larger-files?category_id=29
Successfully uploaded from front end and backend. However in frontend you have a redirect to a strange place after the upload completes.