MM-15480: Add server support for Range on files (#10892)

app/file.go

@@ -81,7 +81,7 @@ func (a *App) ReadFile(path string) ([]byte, *model.AppError) {
 }
 
 // Caller must close the first return value
-func (a *App) FileReader(path string) (io.ReadCloser, *model.AppError) {
+func (a *App) FileReader(path string) (filesstore.ReadCloseSeeker, *model.AppError) {
 	backend, err := a.FileBackend()
 	if err != nil {
 		return nil, err