how to uninstall specific files

The actual items that are installed through the launcher are in the dccache folder in files that end with .ebc. There will also be in the dcbackup folder a file with a string of code like you mentioned, but that is not where the item resides. You can go through your launcher and find the picture of the dress you want to get uninstall, and hit uninstall. Just know that this will not remove the dress from the file that it is in, it will just no longer be recognized in game.