Reorganize file util functionality (#7848)

* reorganize file util functionality

* fix api test compilation

* fix rebase issue

utils/file_backend.go

@@ -0,0 +1,44 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package utils
+
+import (
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/model"
+)
+
+type FileBackend interface {
+	TestConnection() *model.AppError
+
+	ReadFile(path string) ([]byte, *model.AppError)
+	MoveFile(oldPath, newPath string) *model.AppError
+	WriteFile(f []byte, path string) *model.AppError
+	RemoveFile(path string) *model.AppError
+
+	ListDirectory(path string) (*[]string, *model.AppError)
+	RemoveDirectory(path string) *model.AppError
+}
+
+func NewFileBackend(settings *model.FileSettings) (FileBackend, *model.AppError) {
+	switch *settings.DriverName {
+	case model.IMAGE_DRIVER_S3:
+		return &S3FileBackend{
+			endpoint:  settings.AmazonS3Endpoint,
+			accessKey: settings.AmazonS3AccessKeyId,
+			secretKey: settings.AmazonS3SecretAccessKey,
+			secure:    settings.AmazonS3SSL == nil || *settings.AmazonS3SSL,
+			signV2:    settings.AmazonS3SignV2 != nil && *settings.AmazonS3SignV2,
+			region:    settings.AmazonS3Region,
+			bucket:    settings.AmazonS3Bucket,
+			encrypt:   settings.AmazonS3SSE != nil && *settings.AmazonS3SSE && IsLicensed() && *License().Features.Compliance,
+			trace:     settings.AmazonS3Trace != nil && *settings.AmazonS3Trace,
+		}, nil
+	case model.IMAGE_DRIVER_LOCAL:
+		return &LocalFileBackend{
+			directory: settings.Directory,
+		}, nil
+	}
+	return nil, model.NewAppError("NewFileBackend", "No file driver selected.", nil, "", http.StatusInternalServerError)
+}