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

docs/content/docs.md

@@ -1840,6 +1840,12 @@ would be backed up to `file.txt-2019-01-01` and with the flag it would
 be backed up to `file-2019-01-01.txt`.  This can be helpful to make
 sure the suffixed files can still be opened.
 
+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`.
+
 ### --syslog ###
 
 On capable OSes (not Windows or Plan9) send all log output to syslog.

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
 }

fs/operations/operations_test.go

@@ -371,9 +371,14 @@ func TestSuffixName(t *testing.T) {
 		{"test.txt", "-suffix", false, "test.txt-suffix"},
 		{"test.txt", "-suffix", true, "test-suffix.txt"},
 		{"test.txt.csv", "-suffix", false, "test.txt.csv-suffix"},
-		{"test.txt.csv", "-suffix", true, "test.txt-suffix.csv"},
+		{"test.txt.csv", "-suffix", true, "test-suffix.txt.csv"},
 		{"test", "-suffix", false, "test-suffix"},
 		{"test", "-suffix", true, "test-suffix"},
+		{"test.html", "-suffix", true, "test-suffix.html"},
+		{"test.html.txt", "-suffix", true, "test-suffix.html.txt"},
+		{"test.csv.html.txt", "-suffix", true, "test-suffix.csv.html.txt"},
+		{"test.badext.csv.html.txt", "-suffix", true, "test.badext-suffix.csv.html.txt"},
+		{"test.badext", "-suffix", true, "test-suffix.badext"},
 	} {
 		ci.Suffix = test.suffix
 		ci.SuffixKeepExtension = test.keepExt