SOLIDPOD: Provide a function to check if a file is encrypted

[tonypioneer]

Description

Currently solidpod does not require the .enc.ttl suffix for encrypted files. It is not straightforward to check if a file is encrypted at the moment. Therefore, it would be better if solidpod could provide a function to check if a file is encrypted.

Originally posted by @cdawei in anusii/solidui#196 (comment)