Add support for webp images to insert image flow

[jpd236]

Uploading .webp images via the "Insert image" flow seems to fail with an error in Document.methods.insertImage:

FetchError · invalid json response body at https://script.google.com/[snip] reason: Unexpected token < in JSON at position 0

A couple of users hit this with our test run via Galactic Puzzle Hunt. If it's not too much trouble, it seems like it would be nice to support this as it's becoming more prevalent.