Hi Vishnu,
If you are populating any field on the target side by maintaining the validations on the filename field present on the source side, then you can go with variable substitution option available onthe receiver adapter. But if you are using any content conversion to convert xml file to a flat file, then this variable substitution does not work.
In this case, you need to use a udf in the mapping level. Please activate ASMA in the receiver file channel.
Please let me know if you want any help on the udf part.