To edit file information for multiple files browse to your archive (when logged in as a contributor or admin) and select multiple files by using click + shift (click on a file to select it, then hold down the shift button and click on another file. This will select the two files you have clicked on and all those between).
The file information (file info) fields are displayed on the right of the screen.
You can write new file information to multiple files, edit existing data (where the data is identical for all files) or 'add' file information to files that already have information.
For example you might have written specific captions for a batch of files but wish to add a generic paragraph to all of them. You can do this by selecting the files and then 'adding' a few lines of text. This screenshot illustrates the position of the 'add' option which is selected by default to prevent accidental replacement of file info/metadata.