$WBCID is available for use with mail-related WBSP commands (P and L):
filename - the name of the file embedded to the mail message (using WB_Embed variable).
The CID (content id) value of filename generated during the embedding process. Please note that filename argument of $WBCID function must be exactly the same as file name used in the WB_Embed variable.
If you change $wbcid[/poster.jpg] to $wbcid[poster.jpg] in the following example, the function will not return proper CID even if "/poster.jpg" and "poster.jpg" are both referencing the same file (e.g. both WBSP file and poster.jpg are located in the document root).