REST File Upload should explain specific policies required for uploading a file

[gorbunkov]

Jmix docs section: https://docs.jmix.io/jmix/rest/files-api.html

A role with rest.fileUpload.enabled specific policy must be assigned to user in order to upload files.

A role with rest.fileDownload.enabled specific policy must be assigned to user in order to download files.

It is also worth mentioning that a spring boot property for uploaded file limit must be set, e.g.

spring.servlet.multipart.max-file-size=10MB

See forum topic.