Fix bookmark fileinput

[wch]

Fixes #1368. The problem was that a restored fileInput wasn't marked with the correct serializer because it uses a different code path from a file that was uploaded the normal way.

This PR also:

• Copies the restored file to a temp dir. Previosuly, it let the restored session access the bookmarked file directly, which could be a problem if the user's code modifies the file.
• Has a better check that the restored file doesn't contain a path, so that a malicious user can't access any other parts of the filesystem.
• Uses createUniqueId in the FileUploadContext, for consistency.

[wch]

This is a separate issue, but it seems to me that the id shouldn't be recorded here. Instead, it should be recorded after the dir.create() succeeds. Although realistically, there's no need to attempt the dir.create and then loop if it fails, because the probability of a name collision is so low. It seems more likely that the dir.create would fail due to a permission problem, and then this code would get stuck in an infinite loop.