It works for me, but I'm not by a computer at the moment and I'm away for a week so I'm afraid I can't be of immediate assistance.
I also can't remember why I introduced NAExternalFile as opposed to just using MAExternalFileModel - but I do remember making the decision. I seem to remember writing some comments explaining the reason - if we're lucky class comments, otherwise they will be somewhere in the code.
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"I represent an external file saved on the disk not in memory. I differ from ==MAExternalFileModel== as I don't deal with saving a file contents and generating a unique location, I only read files and don't write them. I expect to be passed a filepath to an already saved file.
I also store a url used to retrieve the file via a web server."