( resource pxdoc, string filename )
Sets the name of the file where blobs are going to be read from. Without
calling this function, px_get_record() will only return
data in blob fields if the data is part of the record and not stored in the
blob file. Blob data is stored in the record if it is small enough to fit
in the size of the blob field.
Calling this function twice will close the first blob file and open the new