Try grabbing Content-Disposition from a HEAD request for the save link completion list. Localify descriptions.

common/locale/en-US/messages.properties

@@ -64,6 +64,12 @@ buffer.noClosed = No matching closed tab
 buffer.noAlternate = No alternate page
 buffer.backgroundLoaded = Background tab loaded: %S
 
+buffer.save.altText   = Alternate Text
+buffer.save.filename  = File Name
+buffer.save.linkText  = Link Text
+buffer.save.pageName  = Page Name
+buffer.save.suggested = Server-suggested Name
+
 # TODO: categorise these
 buffer.noTitle = [No Title]
 buffer.noName = [No Name]