Image upload disappears

I'm using plugins_hooked_nav to add a new page to the admin plugin to manage blog authors. The page uses a standard form blueprint which includes a drag and drop image upload. When adding an image it appears to upload normally (does the loading bar and tick icon). Then when I submit the form to save the rest of the data entered, the page reloads and all the text input is saved, but the image is not there (and not in the specified location of user/images). Code is here: https://github.com/matt-j-m/grav-plugin-aura-authors/tree/admin-panel