Reorganize file util functionality (#7848)

* reorganize file util functionality

* fix api test compilation

* fix rebase issue

einterfaces/brand.go

@@ -12,13 +12,3 @@ type BrandInterface interface {
 	SaveBrandImage(*multipart.FileHeader) *model.AppError
 	GetBrandImage() ([]byte, *model.AppError)
 }
-
-var theBrandInterface BrandInterface
-
-func RegisterBrandInterface(newInterface BrandInterface) {
-	theBrandInterface = newInterface
-}
-
-func GetBrandInterface() BrandInterface {
-	return theBrandInterface
-}