When spraying BLOBs, default prefix to FILENAME,FILESIZE

If spraying blobs and no prefix is defined, assume FILENAME,FILESIZE.

Spraying a set of files as blobs without any prefix makes little sense, you need to at least know the size of each blob. Conventionally each blob is prefixd with a variable length filename, followed by a 4 byte blob size.
Default to this when none is specified.