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This is what I get after attaching a file and pressing "Post Discussion"
I'm running Vanilla 2.6.4 on an Ubuntu server running version16.04.5 LTS (Xenial Xerus) and PHP version 7.1.20
The FileUpload plugin comes with a MediaModel which doesn't respect the DatasetType and therefore returns an Object instead of the Array that is expected.
Deactivate the FileUpload plugin (not needed when using the Advanced Editor) and if it does not work right after that, delete the files in /cache