One of the requirements for sharing attachments and library files is a shared folder open to all SalesLogix users.  Security on this folder should be set to full control.  We usually test the shared folder, slxlogging for our example, by asking users to browse to the folder, open a file, write a file to the folder, and delete a file from the folder.  If you are running synchronization and using a service account to run the the sync service make sure that account has full access to the slxlogging folder.    There was a recent case where the Sharing option was missing from the right click pull down menu.   We discovered this system was using FAT32 file format instead of NTFS and we ended up converting the drive to NTFS.  Folder sharing may look simple but if it's not done properly you will run into issues with attachments and sync.