[#188] Improve content-type detector

Signed-off-by: Aleksey Kravchenko <al.kravchenko@yadro.com>

internal/handler/reader.go

@@ -4,9 +4,11 @@ import (
 	"bytes"
 	"context"
 	"io"
+	"mime"
 	"net/http"
 	"path"
 	"strconv"
+	"strings"
 	"time"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/logs"
@@ -25,7 +27,7 @@ type readCloser struct {
 
 // initializes io.Reader with the limited size and detects Content-Type from it.
 // Returns r's error directly. Also returns the processed data.
-func readContentType(maxSize uint64, rInit func(uint64) (io.Reader, error)) (string, []byte, error) {
+func readContentType(maxSize uint64, rInit func(uint64) (io.Reader, error), filename string) (string, []byte, error) {
 	if maxSize > sizeToDetectType {
 		maxSize = sizeToDetectType
 	}
@@ -44,7 +46,20 @@ func readContentType(maxSize uint64, rInit func(uint64) (io.Reader, error)) (str
 
 	buf = buf[:n]
 
-	return http.DetectContentType(buf), buf, err // to not lose io.EOF
+	contentType := http.DetectContentType(buf)
+
+	// Since the detector detects the "text/plain" content type for various types of text files,
+	// including CSS, JavaScript, and CSV files,
+	// we'll determine the final content type based on the file's extension.
+	if strings.HasPrefix(contentType, "text/plain") {
+		ext := path.Ext(filename)
+		// If the file doesn't have a file extension, we'll keep the content type as is.
+		if len(ext) > 0 {
+			contentType = mime.TypeByExtension(ext)
+		}
+	}
+
+	return contentType, buf, err // to not lose io.EOF
 }
 
 type getMultiobjectBodyParams struct {
@@ -128,7 +143,7 @@ func (h *Handler) receiveFile(ctx context.Context, req request, objAddress oid.A
 
 		contentType, payloadHead, err = readContentType(payloadSize, func(uint64) (io.Reader, error) {
 			return payload, nil
-		})
+		}, filename)
 		if err != nil && err != io.EOF {
 			req.log.Error(logs.CouldNotDetectContentTypeFromPayload, zap.Error(err))
 			response.Error(req.RequestCtx, "could not detect Content-Type from payload: "+err.Error(), fasthttp.StatusBadRequest)