1. This is because of how Linux treats things (EVERYTHING is a file). Change the name of the directory or file. So unless you want to change how Linux and in proxy Unix works, this is unlikely to change. Just rename the directory by using a capital letter or even adding a letter or number. Before you bitch about it, consider how Windows treats files. You can only use one name once regardless of case. Hence NOT A BUG.
2. Just don't check the "Use as default application" box when selecting something. This is related to PCManFM and can easily be corrected by editing /usr/share/applications/mineinfo.cache and changing the order to have you prefered application first:
image/x-png=viewnior.desktop;mtpaint.desktop;
It's not related to SliTaz, so technically not our bug.