diff --git a/importer.go b/importer.go
index 4d1802f..d137e94 100644
--- a/importer.go
+++ b/importer.go
@@ -303,23 +303,8 @@ func guessFormat(fileName string) Format {
 			return CSV
 		}
 		ext := strings.ToUpper(strings.TrimLeft(dotExt, "."))
-		switch ext {
-		case "CSV":
-			return CSV
-		case "TSV":
-			return TSV
-		case "PSV":
-			return PSV
-		case "LTSV":
-			return LTSV
-		case "JSON", "JSONL":
-			return JSON
-		case "YAML", "YML":
-			return YAML
-		case "TBLN":
-			return TBLN
-		case "WIDTH":
-			return WIDTH
+		if format, ok := extToFormat[ext]; ok {
+			return format
 		}
 		fileName = fileName[:len(fileName)-len(dotExt)]
 	}
diff --git a/input_csv.go b/input_csv.go
index fd6f8c6..d339ff6 100644
--- a/input_csv.go
+++ b/input_csv.go
@@ -90,6 +90,16 @@ func NewCSVReader(reader io.Reader, opts *ReadOpts) (*CSVReader, error) {
 	return r, nil
 }
 
+func NewTSVReader(reader io.Reader, opts *ReadOpts) (*CSVReader, error) {
+	opts.InDelimiter = "\t"
+	return NewCSVReader(reader, opts)
+}
+
+func NewPSVReader(reader io.Reader, opts *ReadOpts) (*CSVReader, error) {
+	opts.InDelimiter = "|"
+	return NewCSVReader(reader, opts)
+}
+
 func (r *CSVReader) setColumnType() {
 	if r.names == nil {
 		return
diff --git a/reader.go b/reader.go
index da46c03..efec33b 100644
--- a/reader.go
+++ b/reader.go
@@ -3,8 +3,65 @@ package trdsql
 import (
 	"io"
 	"log"
+	"sync"
 )
 
+// extToFormat is a map of file extensions to formats.
+var extToFormat map[string]Format = map[string]Format{
+	"CSV":   CSV,
+	"LTSV":  LTSV,
+	"JSON":  JSON,
+	"JSONL": JSON,
+	"YAML":  YAML,
+	"YML":   YAML,
+	"TBLN":  TBLN,
+	"TSV":   TSV,
+	"PSV":   PSV,
+	"WIDTH": WIDTH,
+}
+
+// ReaderFunc is a function that creates a new Reader.
+type ReaderFunc func(io.Reader, *ReadOpts) (Reader, error)
+
+// readerFuncs maps formats to their corresponding ReaderFunc.
+var readerFuncs = map[Format]ReaderFunc{
+	CSV: func(reader io.Reader, opts *ReadOpts) (Reader, error) {
+		return NewCSVReader(reader, opts)
+	},
+	LTSV: func(reader io.Reader, opts *ReadOpts) (Reader, error) {
+		return NewLTSVReader(reader, opts)
+	},
+	JSON: func(reader io.Reader, opts *ReadOpts) (Reader, error) {
+		return NewJSONReader(reader, opts)
+	},
+	YAML: func(reader io.Reader, opts *ReadOpts) (Reader, error) {
+		return NewYAMLReader(reader, opts)
+	},
+	TBLN: func(reader io.Reader, opts *ReadOpts) (Reader, error) {
+		return NewTBLNReader(reader, opts)
+	},
+	TSV: func(reader io.Reader, opts *ReadOpts) (Reader, error) {
+		return NewCSVReader(reader, opts)
+	},
+	PSV: func(reader io.Reader, opts *ReadOpts) (Reader, error) {
+		return NewCSVReader(reader, opts)
+	},
+	WIDTH: func(reader io.Reader, opts *ReadOpts) (Reader, error) {
+		return NewGWReader(reader, opts)
+	},
+}
+
+var extFormat Format = 100
+var registerMux = &sync.Mutex{}
+
+func RegisterReaderFunc(ext string, readerFunc ReaderFunc) {
+	registerMux.Lock()
+	defer registerMux.Unlock()
+	extToFormat[ext] = extFormat
+	readerFuncs[extFormat] = readerFunc
+	extFormat++
+}
+
 // Reader is wrap the reader.
 // Reader reads from tabular files.
 type Reader interface {
@@ -12,7 +69,7 @@ type Reader interface {
 	Names() ([]string, error)
 	// Types returns column types.
 	Types() ([]string, error)
-	// PreReadRow is returns only columns that store preread rows.
+	// PreReadRow is returns only columns that store preRead rows.
 	PreReadRow() [][]interface{}
 	// ReadRow is read the rest of the row.
 	ReadRow(row []interface{}) ([]interface{}, error)
@@ -157,28 +214,12 @@ func NewReader(reader io.Reader, readOpts *ReadOpts) (Reader, error) {
 	if reader == nil {
 		return nil, ErrNoReader
 	}
-	switch readOpts.realFormat {
-	case CSV:
-		return NewCSVReader(reader, readOpts)
-	case TSV:
-		readOpts.InDelimiter = "\t"
-		return NewCSVReader(reader, readOpts)
-	case PSV:
-		readOpts.InDelimiter = "|"
-		return NewCSVReader(reader, readOpts)
-	case LTSV:
-		return NewLTSVReader(reader, readOpts)
-	case JSON:
-		return NewJSONReader(reader, readOpts)
-	case YAML:
-		return NewYAMLReader(reader, readOpts)
-	case TBLN:
-		return NewTBLNReader(reader, readOpts)
-	case WIDTH:
-		return NewGWReader(reader, readOpts)
-	default:
+	readerFunc, ok := readerFuncs[readOpts.realFormat]
+	if !ok {
 		return nil, ErrUnknownFormat
 	}
+
+	return readerFunc(reader, readOpts)
 }
 
 func skipRead(r Reader, skipNum int) {
diff --git a/trdsql.go b/trdsql.go
index 8e44e67..742d1d1 100644
--- a/trdsql.go
+++ b/trdsql.go
@@ -92,11 +92,11 @@ const (
 	YAML
 
 	// import
-	// Tab-Separated Values format.
+	// Tab-Separated Values format. Format using go standard CSV library.
 	TSV
 
 	// import
-	// Pipe-Separated Values format.
+	// Pipe-Separated Values format. Format using go standard CSV library.
 	PSV
 )