Increasing max files or folders per directory on Linux EXT filesystem

If a (poorly coded) app reaches the maximum number of files or folders per directory in Linux, you may see errors like this:

Error happened when generating Download Link.
Please try again or Contact administrator.
(ERROR:mkdir)

A quick and dirty way to increase the limit (and overall performance of the system) is to add the dir_index flag to the ext filesystem, then reindex:

tune2fs -O dir_index /dev/sda3
updatedb &