{{#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: 10 voluntaries for organ or Harpsichord | |Sonata (piano) |Instrumental (chamber) | | | | | |Baroque | |organ or harpsichord | }}