From time to time, you may find it helpful to copy the filenames of a specific set of photos in a Gallery. For example, if you wanted to edit your client's favorite photos, you may want to copy the filenames of a favorites submission and paste them in Lightroom, so you don't have to match them up one by one.
To copy the filename(s) of whichever photo(s) you have selected, select the photo(s) and click the "Copy Filename" button in the top bar of the "Photos" panel while editing a Gallery.
To select multiple photos, you can:
Click on one photo and then command-click on each other photo.
Click on one photo and then shift-click on another photo. This will select every photo between the first and last one.
Note: In order to have your images filter properly in Lightroom, please make sure that your filenames do not have spaces (Image-01, not Image 01).