If you have uploaded a file and need to assign (or reassign) a filter, select the document, then choose the assign option for filters and you will see this window to allow you to select the appropriate filter. By selecting one document and holding down the shift or control key you can select multiple documents to enable assigning filters in bulk.

It is possible to create, rename and delete filters - you can only rename and delete filters that you have created. Any documents under a filter that is deleted will move up one level to the next highest filter.