If the mimetype is missing in metadata, glideURL won't work

[WesleyE]

On many drivers, the metadata does not contain the mimetype. This is the case for the current version of the local driver. This results in the Driver skipping the urlBuilder part.
In laravel-elfinder however, if the mimetype is null it again tries to guess the mimetype. This might explain why the issue has not come up before.
See PR #50