[w3c/FileAPI] Specify Data URL media-type as `application/octet-stream` when `mimeType` not available (PR #220)

While implementing FileAPI in Servo, https://github.com/servo/servo/pull/44897
I notice we fail to pass some readAsDataURL WPT tests: 
1. [result for Blob with unspecified MIME type](https://github.com/servo/servo/blob/1490ec70d422ee69ead5571a1c4ae93d92f0b07a/tests/wpt/tests/FileAPI/reading-data-section/filereader_readAsDataURL.any.js#L42)
2. [readAsDataURL result for empty Blob](https://github.com/servo/servo/blob/1490ec70d422ee69ead5571a1c4ae93d92f0b07a/tests/wpt/tests/FileAPI/reading-data-section/filereader_readAsDataURL.any.js#L54)

It seems that spec is not aligned with test expectations. WebKit, Chromium, Gecko would fallback to `application/octet-stream`.

Part of #104

Implementation commitment:

 * [x] WebKit
 * [x] Chromium
 * [x] Gecko
And Servo will do this from now on.


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  https://github.com/w3c/FileAPI/pull/220

-- Commit Summary --

  * Specify Data URL media-type as `application/octet-stream` when `mimeType` not available

-- File Changes --

    M index.bs (2)

-- Patch Links --

https://github.com/w3c/FileAPI/pull/220.patchhttps://github.com/w3c/FileAPI/pull/220.diff
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