{{#imslpmsg:addfile}} Use a pipe after addfile to separate, then put the number by which you want to increment the file number displayed; for example, {{Addfile|1}} would increase the file number displayed by 1 (i.e. one cross-referenced file in addition to the files that are there).
{{#imslpworkentry: 6 Arabesques, Op.67 | |Piano piece |Instrumental (chamber) |Op.67 | |6 | |A mon ami F. Blumenfeld |Romantic | | | }}