custom metabox type file only shows some images from Media Library

I’m building a site using a local install of 4.5.3
I’ve created a custom metabox to allow me to add an image to a post:

array(
			'name' => 'Source image',
			'desc' => 'Upload the source image',
			'id' => 'crcr_br_source_image',
			'type' => 'file'
		),

When I either edit a post or add a new post the metabox displays as expected. However when I click ‘add file’ and choose the Media Library tab, I only see a few of the images that are in my Media Library. The images that I can see are all mp3 files – none of my jpg files are displayed. Where am I going wrong?