To assign the app licenses to users, you need to follow the steps -
- From Setup, enter Installed Packages in the Quick Find box, then select Installed Packages to find the installed package that has available licenses.

- Click the Manage Licenses link next to the Mass File Upload name.

- Click "Add Users".

- Choose a view from the drop-down list, or click "Create New View" to build a new custom view.
- Click a letter to filter the users with the last name that corresponds with that letter, or click All to display all users who match the criteria of the current view.
- Select users.
- To select individual users, use the checkboxes. Selected users are listed in the Selected list. When the list includes all the users to whom you want to assign licenses, click Add.
- To select all users for the current view, click the Add All Users button, then click OK.

PS: View Mass File Upload, Salesforce file upload app, on Salesforce AppExchange:
https://appexchange.salesforce.com/appxListingDetail?listingId=82cf236b-1ee8-4ea7-aebf-fd0c89fb93bd