Datapump Changes Case of Supplied Parameter

When using datapump for nightly full exports, I get the datyofweek from command prompt thusly:

C:\> echo on

C:\>set PARSEARG="eol=; tokens=1,2,3,4* delims=/, "

C:\>for /F %PARSEARG% %i in ('DATE/T') do set DAYOFWEEK=%i

C:\>set DAYOFWEEK=Fri

This value is passed to datapump:

expdp.exe system/pwd@SID   DIRECTORY=expdp_dir DUMPFILE=SID_expfull_%DAYOFWEEK%.dmp  FULL=y LOGFILE=SID_expfull_%DAYOFWEEK%.log  REUSE_DUMPFILES=y

Even though the dumpfile file name is passed to expdp as MIXED case, when datapump creates the dumpfile, it converts the filename case to UPPER.  Please note that the logfile specification (also mixed case) is unaffected.