Thank you all very much for your help!
I contacted my hoster - and I think, there was changed something about the permissions today, because any changings in my files are working since today when I upload my files again.
I was a bit confused before, therefore I mixed things up:
All files I had changed during the last days had been overwritten by uploading them via FTP, because I can see my changings if I download those files again. But the changings did not work and they also were not shown to me in the "Console".
Emanuele, I do not have a mobile phone, I am working with an old pc - and I think something like filezilla would be a good opinion for me.
Your code for the centered attachments is working splendit, everything is looking nice and proper again.
I changed it that way, so a few of smaller attachments can stand side by side in one row:
.postarea .attachments .attachment {
display: inline-block;
}
.postarea .attachments .attachment_name {
display: block;
line-height: 1.3em;
}
Thank you for your help!