Same deal as with the File and Description templates... this is now basically a wrapper. Use it where a cross-referenced file (i.e. the link is on the page but not the file itself) does not give an accurate number of files on the tab; especially where there are no actual files, for example, in the Arrangements tab, and only a link, the displayed number on the tab will be 0, which will make people think there is no point clicking on that tab.
{{#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).
