Renaming lots of files at once

I work as a freelance video editor and frequently receive large numbers of video clips that are supposedly in .mp4 format but they don’t have the file extension, so lots of apps don’t recognise them. If I rename the file to add the extension myself they work fine but there are lots of them and this can take a long time. Is there a utility that will put the right extension on them for me automatically? Lynn Ponting Yosemite added a handy batch file renaming function that might do what you need. Just select multiple files, right-click and choose Rename from the context menu. Change ‘Replace Text’ to ‘Add Text’ and put ‘.mp4’ in the box next to it. This works very well for most files but it will fail for those that have full stops in the filename, such as ProjectX.Source.Clip1. Using batch rename on this will turn it into ProjectX.Source.mp4. Clip1 because Yosemite interprets the fragment after the last full stop as the proper file extension and is trying to preserve that. For a more bulletproof solution, you’ll need to use the Terminal. Type followed by a space and drag the folder containing your files from Finder to the Terminal window. This supplies the full path to the cd command. Press ® to change to that folder. Next, enter the command:

to add .mp4 to the end of every filename in that folder.