you have to add the item into your "storage" on mpgh, then you select the item that you put your item in. basically there's kind of a middleman system when inserting attachments. it goes like
your file>MPGH storage>attachment
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yeah, here. scroll down, click on "Manage Attachments" selection, then you should get a pop-up. in the pop-up, you click on "Add Files +" then you add your files, and then select them and click "Insert Inline"