.MIGRATEDPHOTOLIBRARY File Extension
What is a MIGRATEDPHOTOLIBRARY file?
Library of digital photos created by iPhoto that has been migrated to Photos, an image organization application that replaced iPhoto; contains all of the images that appeared in the discontinued iPhoto application along with the edits that have been made to them.
When Photos successfully migrates a iPhoto library, the "photolibrary" file extension is changed to"migratedphotolibrary" and Photos makes a hard link to all of the iPhoto media assets. Photos uses the same directory structure as iPhoto.
NOTE: Photos performs a similar function with Aperture libraries but uses the .MIGRATEDAPLIBRARY file.