You can write the file to an intermediate directory & when it's complete move it to WMB's input directory. How you detect the file's complete & move it is an exercise for the reader, as is how ...
I happen a issue about SAP transferring a large file to WMB,
I use FileInput Node receiver from SAP message,
when SAP create a flat file(IDOC) and append data to this file,
then large data not app ...