2022.1

It is possible to format the number using a pattern and locale. See
"Formatting date and number values " on page1556, below.
${template.ext}
The extension that corresponds to the chosen output format (in
lower case).
For example, for PDF output, ${template.ext} would be pdf, for
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1556,
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.
Page 1550