PSB (Photoshop's Large Document Format) is an image file format supported by Adobe Photoshop CS and later versions of Photoshop. It is a companion format to PSD. PSB supports larger image sizes (up to 300,000×300,000 pixels), and increases various other limits. PSB is nearly identical to PSD, except that some fields are larger.
PSB files begin with bytes
'8' 'B' 'P' 'S' 0x00 0x02.