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In WF preferences, under File Submission, you need to control-click 'Webmasters' and 'Registered Users' (or anonymous users) to ensure they are both on the list. You then set the max filesize etc in the fields directly below.
In Mimetype Management, you then need to find the mimetype of the files you want to allow users to upload. Click the 'Red' button in the 'User' column to turn it 'Green'. The mimetype(s) will appear in the list called 'Allowed User Mimetypes'
When a user is in the WF-Sections 'section', there should be a button in the menubar titled 'Submit Article'. About half way down the submit article form, under 'Create Upload' there should be a filename field and a 'Browse' button for you to locate the file to upload.
AFAIK