Restrict file types

[danyill]

It would be helpful to me if the open plugin accepted files similar to openscd.github.io:

https://github.com/openscd/open-scd/blob/9094855c03180d3ee30319df930179b5c7b4650c/packages/plugins/src/menu/OpenProject.ts#L37-L41

Specifically, .sed,.scd,.ssd,.isd,.iid,.cid,.icd