Hi Simon,
This is totally awesome! I have been uploading photos recently to Commons and was wondering how I would be able to geotag them efficiently. I tested the tool a while ago and it works as expected.
Here are some suggestions:
1. The coordinates have too much precision. I see 15 decimal places in the tool, which makes the photos geocoded to an accuracy of one-tenth of a nanometer. What is ultimately stored into the File page is 10 decimal places which is still too precise (~10 μm). 5 decimal places (~1 meter accuracy) should be more than enough.
2. For the step of selecting files, it would also help to have the option of specifying a user, defaulting to the OAuth-authenticated user. Usually you would want to geocode files that you yourself uploaded.