The Place dialog classed every .pdf, .ai and .ait path as a document
that Link places as one locked object. A .ai saved without PDF
compatibility is PostScript, and file.place embeds it as editable art
with Link on or off, so the tooltip was wrong for it. The dialog now
reads the file's header with the same PostScript check file.place uses,
and describes such a file as it does other files Link does not keep
linked.