Thinking about it, I am not sure that Kindle for PC provides an automated way of side-loading from local file store. What I have done in the past is double-clicked .mobi file in the Send to Folder location, and that has promptly opened the file in the Kindle app and transferred it into the Kindle library. I do not really use Kindle for PC in anger as I mainly read books on my iPad, so there may be a better and more automated way that I am not aware of.
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