From my reading of RFC 2048 and https://mimesniff.spec.whatwg.org/#parsing-a-mime-type, empty quoted strings are acceptable for Content-Type parameter values.
Created attachment 354118 [details] Patch
Comment on attachment 354118 [details] Patch Attachment 354118 [details] did not pass mac-ews (mac): Output: https://webkit-queues.webkit.org/results/9897056 New failing tests: imported/w3c/web-platform-tests/xhr/overridemimetype-blob.html
Created attachment 354131 [details] Archive of layout-test-results from ews102 for mac-sierra The attached test failures were seen while running run-webkit-tests on the mac-ews. Bot: ews102 Port: mac-sierra Platform: Mac OS X 10.12.6
Comment on attachment 354118 [details] Patch Attachment 354118 [details] did not pass mac-wk2-ews (mac-wk2): Output: https://webkit-queues.webkit.org/results/9897391 New failing tests: imported/w3c/web-platform-tests/xhr/overridemimetype-blob.html
Created attachment 354135 [details] Archive of layout-test-results from ews107 for mac-sierra-wk2 The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews. Bot: ews107 Port: mac-sierra-wk2 Platform: Mac OS X 10.12.6
Comment on attachment 354118 [details] Patch Attachment 354118 [details] did not pass ios-sim-ews (ios-simulator-wk2): Output: https://webkit-queues.webkit.org/results/9897096 New failing tests: imported/w3c/web-platform-tests/xhr/overridemimetype-blob.html
Created attachment 354136 [details] Archive of layout-test-results from ews125 for ios-simulator-wk2 The attached test failures were seen while running run-webkit-tests on the ios-sim-ews. Bot: ews125 Port: ios-simulator-wk2 Platform: Mac OS X 10.13.6
Comment on attachment 354118 [details] Patch Attachment 354118 [details] did not pass mac-debug-ews (mac): Output: https://webkit-queues.webkit.org/results/9897632 New failing tests: imported/w3c/web-platform-tests/xhr/overridemimetype-blob.html
Created attachment 354146 [details] Archive of layout-test-results from ews113 for mac-sierra The attached test failures were seen while running run-webkit-tests on the mac-debug-ews. Bot: ews113 Port: mac-sierra Platform: Mac OS X 10.12.6
Created attachment 354213 [details] Patch
Created attachment 354322 [details] Patch
Created attachment 354341 [details] Patch
Created attachment 354457 [details] Patch
Comment on attachment 354457 [details] Patch Attachment 354457 [details] did not pass mac-ews (mac): Output: https://webkit-queues.webkit.org/results/9939817 New failing tests: imported/w3c/web-platform-tests/mimesniff/mime-types/charset-parameter.window.html imported/w3c/web-platform-tests/xhr/overridemimetype-blob.html imported/w3c/web-platform-tests/mimesniff/mime-types/parsing.any.html imported/w3c/web-platform-tests/mimesniff/mime-types/parsing.any.worker.html
Created attachment 354459 [details] Archive of layout-test-results from ews100 for mac-sierra The attached test failures were seen while running run-webkit-tests on the mac-ews. Bot: ews100 Port: mac-sierra Platform: Mac OS X 10.12.6
Comment on attachment 354457 [details] Patch Attachment 354457 [details] did not pass mac-wk2-ews (mac-wk2): Output: https://webkit-queues.webkit.org/results/9939839 New failing tests: imported/w3c/web-platform-tests/mimesniff/mime-types/charset-parameter.window.html imported/w3c/web-platform-tests/mimesniff/mime-types/parsing.any.html imported/w3c/web-platform-tests/xhr/overridemimetype-blob.html imported/w3c/web-platform-tests/mimesniff/mime-types/parsing.any.worker.html
Created attachment 354462 [details] Archive of layout-test-results from ews105 for mac-sierra-wk2 The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews. Bot: ews105 Port: mac-sierra-wk2 Platform: Mac OS X 10.12.6
Comment on attachment 354457 [details] Patch Attachment 354457 [details] did not pass ios-sim-ews (ios-simulator-wk2): Output: https://webkit-queues.webkit.org/results/9939969 New failing tests: imported/w3c/web-platform-tests/mimesniff/mime-types/charset-parameter.window.html imported/w3c/web-platform-tests/mimesniff/mime-types/parsing.any.html imported/w3c/web-platform-tests/xhr/overridemimetype-blob.html imported/w3c/web-platform-tests/mimesniff/mime-types/parsing.any.worker.html
Created attachment 354465 [details] Archive of layout-test-results from ews122 for ios-simulator-wk2 The attached test failures were seen while running run-webkit-tests on the ios-sim-ews. Bot: ews122 Port: ios-simulator-wk2 Platform: Mac OS X 10.13.6
Comment on attachment 354457 [details] Patch Attachment 354457 [details] did not pass mac-debug-ews (mac): Output: https://webkit-queues.webkit.org/results/9939986 New failing tests: imported/w3c/web-platform-tests/mimesniff/mime-types/charset-parameter.window.html imported/w3c/web-platform-tests/mimesniff/mime-types/parsing.any.html imported/w3c/web-platform-tests/xhr/overridemimetype-blob.html imported/w3c/web-platform-tests/mimesniff/mime-types/parsing.any.worker.html
Created attachment 354466 [details] Archive of layout-test-results from ews113 for mac-sierra The attached test failures were seen while running run-webkit-tests on the mac-debug-ews. Bot: ews113 Port: mac-sierra Platform: Mac OS X 10.12.6
Created attachment 354502 [details] Patch
Comment on attachment 354502 [details] Patch Clearing flags on attachment: 354502 Committed r238124: <https://trac.webkit.org/changeset/238124>
All reviewed patches have been landed. Closing bug.
<rdar://problem/46019534>