It seems fairly common to call overrideMimeType before open(), but should throw an InvalidStateError according to current XHR2 editors draft.
If you search (Google) for overrideMimeType a significant number of first page results show calling of overrideMimeType before the open call, so I imagine this will become increasingly common as people copy+paste. I don't think this is a particularly problematic thing to allow so can we please update the spec to allow this? -- Andrew Oakley
