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).