I think this is a good idea. I think a seperate download button will make things a bit clearer for users that haven't used this feature before. You'll just need to take into consideration where this download button should be placed and how easily it can be found at different screen widths.
Maybe a drop down/popup that appears when you hover over the file name?
It should match any other tables that exist in review board. An example would be the dashboard that displays all review requests but before you start implementing this it might be worth asking one of the mentors if there is another table that would be a better reference for you to use.
Have you given some thought to how wide the indent would be for each directory level contained within the zip? You may have to make the default width of the Name column match the string length of the deepest file with the added indents (i.e. if there are 3 levels of directories then the minimum width of the name column should be the width of the longest filename is this directory level with three indents). If you want the Name column width to be fixed for all zips, tarballs or other archive files then you may need to speak with the mentors to figure out the average number of directory levels in zip files to calculate the average number of indents that will be required).
If you are interested in considering an alternate way of displaying the different directory nesting levels you might want to consider subsituting the indent with the relative path for the file as shown below.
test.zip
file.text
directory_name
directory_name/other_file.text
