[#19] Extract uploading logic into a separate package

Signed-off-by: Pavel Korotkov <pavel@nspcc.ru>

uploader/multipart.go

@@ -0,0 +1,41 @@
+package uploader
+
+import (
+	"io"
+	"mime/multipart"
+
+	"go.uber.org/zap"
+)
+
+type MultipartFile interface {
+	io.ReadCloser
+	FileName() string
+}
+
+func fetchMultipartFile(l *zap.Logger, r io.Reader, boundary string) (MultipartFile, error) {
+	reader := multipart.NewReader(r, boundary)
+
+	for {
+		part, err := reader.NextPart()
+		if err != nil {
+			return nil, err
+		}
+
+		name := part.FormName()
+		if name == "" {
+			l.Debug("ignore part, empty form name")
+			continue
+		}
+
+		filename := part.FileName()
+
+		// ignore multipart/form-data values
+		if filename == "" {
+			l.Debug("ignore part, empty filename", zap.String("form", name))
+
+			continue
+		}
+
+		return part, nil
+	}
+}