impr: backward-compatible compliance with rfc 2397

internal/fileio/fileio.go

@@ -72,24 +72,31 @@ func ReadJson[T any](path string) (T, error) {
 
 // WriteFile writes the file to disk.
 // The content can be text or binary (encoded as a data URL),
-// e.g. data:;base64,MTIz
+// e.g. data:application/octet-stream;base64,MTIz
 func WriteFile(path, content string, perm fs.FileMode) (err error) {
 	var data []byte
-	// TODO: only check for "data:;base64," to comply with RFC 2397.
-	// Remove the "data:" check after the snippet reaches 0.16.
-	if strings.HasPrefix(content, "data:") || strings.HasPrefix(content, "data:;base64,") {
-		// data-url encoded file
-		_, encoded, found := strings.Cut(content, ",")
-		if !found {
-			return errors.New("invalid data-url encoding")
-		}
-		data, err = base64.StdEncoding.DecodeString(encoded)
-		if err != nil {
-			return err
-		}
-	} else {
+	if !strings.HasPrefix(content, "data:") {
 		// text file
 		data = []byte(content)
+		return os.WriteFile(path, data, perm)
+	}
+
+	// data-url encoded file
+	meta, encoded, found := strings.Cut(content, ",")
+	if !found {
+		return errors.New("invalid data-url encoding")
+	}
+
+	if !strings.HasSuffix(meta, "base64") {
+		// no need to decode
+		data = []byte(encoded)
+		return os.WriteFile(path, data, perm)
+	}
+
+	// decode base64-encoded data
+	data, err = base64.StdEncoding.DecodeString(encoded)
+	if err != nil {
+		return err
 	}
 	return os.WriteFile(path, data, perm)
 }