I think you'll be happy with the next release.

It doesn't have sub-categories per see, but it has something new called "Attributes". You can define any number of attributes per repository. Then for each attribute you can define any number of values.

Example: Define an attribute named "Color" with values of "Red", "White" and "Blue". Now when you upload a file to the repository, a set of checkboxes will be auto-generated for each attribute. You can check off any values that you want assigned to that file.  In the header.html template file there is a new [ATTRIBUTE:name] tag.  So for this example, you would add an [ATTRIBUTE:Color] tag to the header. When the repository is displayed, a drop down listbox will be displayed with the possible values for that attribute ( plus an ALL selection ). Selecting a value from the drop will 'filter' the repository. Attribute drop downs are cumulative, so you can have 2 or 3 Attributes and allow the users to filter based on combinations.