HPCC-9271 Show 'truncated' message for viewing large file

There is a 640k limit for viewing a helper file in ECL Workunit
Details page. This fix checks the size of a helper file. If it is
larger than 640k bytes, a message '(Truncated to 640K bytes)' will
be added beside the View File link.

When a helper file is downloaded, the existing code uses 'file path
 + file name' as 'attachment filename'. Internet browser replaces
the Path Separators in the file path with the '_'s and the file
name looks strange. This fix removes the file path from the
'attachment filename'.

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>