setting a base url in file_column

i'm pretty sure i've seen a way to do this before, but i'm not quite
sure i remember where.

basically if i'm using image_tag or link_to around url_for_file_column,
the images work fine. i guess because it automatically adds the base,
but when i use url_for_file_column alone, it doesn't work right because
i have my entire application in a sub folder. is there a dry way to fix
this other than adding the folder name before any place that uses
url_for_file_column?