If there's a limited number of tag categories, then the easiest might be to create a columns menu, where a list of possible categories is given for the user to select from and create a column of. This is the design used by Windows Explorer, iTunes library, etc. Even if the list is extensive, you could determine the most common tag categories, present them in the menu list and expose the remainder through a "More tags..." selection (for example, Windows Explorer presents a list of 10 popular/recent columns to select and clicking "More..." brings up a dialog showing all possible columns - and there are a lot to choose from).
Alternatively (or, probably best, in addition to), you could make the tag category a link within the tag itself:
This gives users a quick way to convert visible tag categories to columns while not adding an awful lot of clutter. You'd have to test to see if people confuse the actions triggered by clicking the category link vs. the rest of the tag. A link title along the lines of "Add column 'Author'" that displays when hovering over the category and "Find similar files tagged 'Author: Florian Cajori'" for the remainder of the tag might help if most users can't make the distinction.
I also added two possible column menu icons that might do well for the menu option I first mentioned - coupling one of them with "Columns..." would clue most users familiar at all with sortable column interfaces into there are more options available to them.