Add file type limitation to uploading a schema

## Background
When I try to upload a schema now to Menas I can choose json, jpeg, xls, etc. but those are not supported files.

## Feature
I would like to limit the files that can be chosen to JSON, json, and I don't know what else we support.

## Proposed Solution [Optional]
[FileUploader](https://sapui5.hana.ondemand.com/#/api/sap.ui.unified.FileUploader) does allow `fileType` specified