Tags can be added to multiple files at once either while you upload or after files have already been uploaded.
To add tags to multiple files while uploading:
Click +Add New.
Click File.
Add a Contact and Account.
Attach multiple files.
Add the file Tags.
Click Upload.
All of the files that have been uploaded will now have the same Tags.
To add tags to multiple files that have already been uploaded:
Click the Files page on the left-hand navigation.
Select all of the files to which you would like to add Tags by clicking the checkbox next to the file name.
Scroll to the bottom of the page and click the Tag icon.
Click Add & Remove Tags.
Click the Tags box and select the Tag you would like to add to the selected files.
Click back into the Tags field to add additional Tags.
Click Add Tag.
Related article: How to Edit File Tags