mime type error on file upload with FS adapter (when only specifying path)

Steps to reproduce the behavior

1. have a model with a field of type File and the file system adapter
2. try to upload a file by calling model._.yourFileField.upload({path: specifiy your path})

Expected behavior

the file appears in the directory specified in the file system adapter

Actual behavior

an error occurs: mime is not a function

it happens because in lib/storage/index.js line 120 the mime type is tried to be evaluated when it wasn't specified in the file parameter, but the call has to be mime.lookup(file.path)