Companion App should provide a configuration option to prevent clearing files on startup

Issue Summary

Currently, on startup the Companion App will remove all attachments from its temporary directory. This is to prevent temporary files building up and taking up too much space. 

However, some users would prefer to have more control over when these files are deleted. We could provide a configuration option in the Companion menu such as "Clear temporary files on startup" that defaults to selected. If the user unticks this option then we would no longer perform the clear out of the temporary directory. 

This suggestion is important in cases when the user did not upload changes, but it has saved them. And because of that, his changes will be gone, and we will not be able to recover them.