I found another little problem, which is possibly limited to my php setup, but I want to post it here, because it maybe is relevant to others anyway.
I am running php with UTF-8 character encoding, I guess it is relevant here. Uploading files via attachments 1.6.x before, seems to be not compatible to UTF-8 and so there now files which have double byte substitutions in filename instead of the original character. For instance there is a "Ã¼" instead of "ü" ans so on. Before update, it worked properly, because attachments plugin just converted it back the same way, so that the wrongly converted filenames are only on the servers side.
Now the new attachments 3.3.2 seems to take care of character encoding or is using UTF-8 (through using the json storage thing). As result, it returns the wrong converted filesnames as is.
So, technically attachments 3 is working better, but on the wrong filenames installed by the old version. The problem will occur on migrating old data only.
For me, it was not so important, because there not so many attachments affected at this time, if there a massive count of attachments with special characters in filename, it could become a mess.