Auto merge of #25227 - gterzian:remove_unnecessary_thread_in_filereader, r=jdm

Remove unnecessary thread in filereader, add stream TODO

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I think there are no benefits to queuing the various `FileReadingTask` from a thread, and reading the actual blob bytes are still done on the main-thread. We could try to move the reading off the main-thread(but note the `Blob` is not something you would want to move to another thread "as-is" since it's a `DomObject`), and I think we should just aim for implementing streams and follow the spec instead.

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