incorrect mime-type on uploaded WXR documents

[pbiron]

When a WXR document is uploaded, the mime-type of attachment post is empty.

This is a not really a bug in this plugin and is actually a result of wp_import_handle_upload() passing $overrides['test_type'] = false to wp_handle_upload(). But my hope is that the feature of this plugin that allows choosing previously uploaded WXR documents from the media library will demonstrate why the bug in wp_import_handle_upload() should be fixed in core.

[pbiron]

I haven't yet created' a trac ticket for this bug in wp_import_handle_upload() because the standard importer isn't really affected by it.

However, with this plugin's feature of selecting previously uploaded WXR documents from the media library the core bug is a problem. So, if this plugin gets to the point where it is ready to supplant the standard import plugin then I'd be happy to open a trac ticket to fix this in core.