DOMFormData is not currently available in workers due FormData (which DOMFormDatas are passed to for conversion to an HTTP body) currently depending on the Document/Page to do file replacement.
As noted, the crux of the issue is that FormData class uses the Page to allow a hosting app to generate a file replacement for things like bundles. This was added in https://trac.webkit.org/changeset/32666/webkit (~9 years ago).
My guess is that we can remove the delegation, and use the now available platform standard ways to detect bundles and generate the replacement.
Indeed, it looks like we have most of the infrastructure in place to do this for Blobs already (File::shouldReplaceFile, File::computeNameAndContentTypeForReplacedFile, BlobDataFileReference::generateReplacementFile).
There is some performance improvement to make potentially - I think that there are situations where we compress the same bundle twice, using the delegate first, and then in the Networking process using WebKit functionality again.