2018.1

PostScript output, ${template.ext} would return ps.
Note that ${template.ext} does not include a leading dot.
File
${file}
${file} is a shorthand for ${file.base}_
${file.nr,0000}.${file.ext}. It expands to an internally
generated file name with a four digit sequence number at the end.
The file extension is determined by the selected output format.
The 0000 in ${file.nr,0000} is a format pattern that takes care of
formatting the number with at least four digits including leading
zero's. See "Formatting date and number values " on page1119,
below.
Example
When printing to PDF, ${file} could expand to
merged.5852941188491153960_0001.pdf.
${file.base} Expands to a unique, internally generated file name, without a
trailing dot or extension.
${file.ext}
The extension that corresponds to the chosen output format, for
example pdf or ps.
Note that ${file.ext} does not include a leading dot.
${file.nr}
An automatic sequence number belonging to the current output file.
It is automatically incremented for each output file that gets created
within the same job.
It is possible to format the number using a pattern and locale. See
"Formatting date and number values " on page1119, below.
${file.pageCount}
This variable was introduced for use in Printer Definitions for
PostScript printers.
As it entails page buffering and could easily lead to Out of Memory
Page 1114