> couldn't replace the attachment
Yes, that can be tricky.
Edit and REMOVE the attachment. "Uncheck the attachments you no longer want attached". Post.
Edit again and attach the new file. Post.
If the newfile has the same name, your -browser- may "assume" it is still the same file, and display the cached copy. Force a full re-load. In Windows IE, Ctrl-F5. (Or give your new file a new name before upload; that's better for others who won't know you recently changed the file.) (The way this forum links attachments, it -may- not be necessary to reload/rename; when I link to external images and get tricky, sometimes I do have to force reload to see the current version.)