Sorry for the long-winded title.
As per my post
here i had an issue with attachments not working after i upgraded to 7.5.6
Now it could be that during the course of the upgrade i messed up the directory permissions on the tmp directory (possible, i did have a few file/dir permission issues).
However, with the tmp permissions not set to writeable, you do not get any error or warning when trying to upload an attachment to a post. Instead all seems to go through okay and you even get your posting back with the attachment link in place.
I guess this could be a rare issue, but thought it worth mentioning.
Regards
Baz