I was recently attempting to use the '/u' for uppercase letters in the filter. I could not get it work as i got 0 files back. I know i have uppercase letters in just about every tag so i should get all files. I then attempted to use the '/l' character, which worked just fine. I am wondering if anyone else could verify that this isn't something that only happens with my installation.
the problem you had with the filter is a limitation of the filter feature. To make the filter more usable for common queries I've decided to always ignore upper- and lowercase.
Internally I'm using both the field content and the filter string in lowercase.