Today, if we're using a Copy Files task and we want to use information regarding the source file (such as it's modified date), There does not appear to be a variable that references the source file.   We have {NEWNAME()} that we can use in the post processing, but if you want to use the modified date of the source file i do not see a currently available variable to use.
For instance,  if my Source Path on the copy step is D:\Temp\SourcePath\ and I want to use the modified date of the source file,  I can't just use {FILE(ModifiedTime | {NEWNAME()}| MMMM yyyy)} because the FILE(ModifiedTime) needs a full path to a file to retrieve that info.
So we'd have to hard code it, like so:  {FILE(ModifiedTime|D:\temp\SourcePath\{NEWNAME()}|MMMM yyyy)}
That would get you the modified date in the format of "MAY 2016" as an example.  And then I could piece together the rest of the file name how i'd like.
Long story short, is there, or can there be added...  a variable to reference the full path to the actively being copied file.  Something like  {SOURCEFILE()}.    
Brian