I'have tested the Nickc25's hints successfully, I want to drill down a little bit more annotated versione of the procedure .
(OSX)
0 - [save],clean-up iMessage attachement on you phone (*1)
1 - Make a backup in iTunes, Exit Itunes
1.1 - Make a safe further copy of this backup on alternate drive (*2)
2 - Use a tool that allows you to browse and change the
backup and delete the 'zombie' files (*3)
I used iBackupBot .
3 - Expand yourphone tree
4 - Select 'Multimedia File Manager'
5 - Click on 'Other Multimedia files' TAB
6 - Scroll down to 'MediaDomain/Library/SMS/Attachments/
Select all , eventually export , delete them
7 - Go back to yourpohone's backup tree
expand 'System Files'/MediaDomain/Library/SMS/Attachments
8 - Go to the list and sort it by Size (Descending)
then delete all non-zero files (*4)
9 - Exit iBackupBot, Open ITunes, Restore this 'edited'
backup, I recovered 256Mb of garbage.
10 - If something gooes wrong and you result in a corrupted
backup, close iTunes, restore the last-good from 1.1,reimage
your phone and retry.
(*1) because we are going to wipe-them-all in the backup
it is a good point to remove-them-all from the phone
first. After that, infacts , the remaining ones on the backup
have to be considered garbage.
IOS6,7 allows you to selectively delete single messages/attachments
within a thread.
(*2) it is recomended to backup this backup, becaus we are going
to 'edit' it with 3rdy-part tools that may eventually corrupt it.
You can trigger a TimeMachine backup or (simpler), copy the folder :
/Volumes/YOURDISK/yourusername/Library/Application Support/MobileSync/Backup/yourdeviceid
somewhere .
(*3) Backups are organized as anonymized files within the DEVID folder
described within a metadata database . All the structure and the contents
are crossreferenced in the metadata database and if you simply modify
the files without updating metadata, we may corrupt the backup .
There are tools around that allows you to browse and edit the backups.
(I used 'iBackupBot' successfully)
(*4)I observed that some more 'zombies' are listed here
despite the action at point 6.