sync: make --suffix-keep-extension preserve 2 part extensions like .tar.gz

If a file has two (or more) extensions and the second (or subsequent)
extension is recognised as a valid mime type, then the suffix will go
before that extension. So `file.tar.gz` would be backed up to
`file-2019-01-01.tar.gz` whereas `file.badextension.gz` would be
backed up to `file.badextension-2019-01-01.gz`

Fixes #6892

fs/operations/operations.go

@@ -618,9 +618,24 @@ func SuffixName(ctx context.Context, remote string) string {
 		return remote
 	}
 	if ci.SuffixKeepExtension {
-		ext := path.Ext(remote)
-		base := remote[:len(remote)-len(ext)]
-		return base + ci.Suffix + ext
+		var (
+			base  = remote
+			exts  = ""
+			first = true
+			ext   = path.Ext(remote)
+		)
+		for ext != "" {
+			// Look second and subsequent extensions in mime types.
+			// If they aren't found then don't keep it as an extension.
+			if !first && mime.TypeByExtension(ext) == "" {
+				break
+			}
+			base = base[:len(base)-len(ext)]
+			exts = ext + exts
+			first = false
+			ext = path.Ext(base)
+		}
+		return base + ci.Suffix + exts
 	}
 	return remote + ci.Suffix
 }