Format description: https://en.wikipedia.org/wiki/Adobe_Photoshop#File_format
Interest: PSD is a broadly used format used to prepare actual media files used on MediaWiki installations, including Wikimedia projects. As Wikimedia communities doesn't seem to be currently interested by proprietary format, the feature would so be primarily for third party MediaWiki installations.
Suggested technical plan: Write a MediaWiki extension to provide an handler for this format. ImageMagick can convert PSD to PNG or JPG, so we can generate thumbnails easily. There are security concerns and so a need to validate uploaded files.