Fix testserver to return a correct MIME-type header.
Fix testserver to return a correct MIME-type header.

Originally, testserver was sending "Content-type: text/html" header regardless
of a file extension, due to a bug in TestPageHandler.GetMIMETypeFromName.

Patch by Yuta Kitamura <yutak@google.com>.
Original code review: <http://codereview.chromium.org/100357>

Review URL: http://codereview.chromium.org/115058

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@15510 0039d316-1c4b-4281-b951-d872f2087c98

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