PY3 MimeTypes wants files in text mode.

See http://hg.python.org/cpython/file/2766320bdb10/Lib/mimetypes.py#l202

before: 455 failed, 248 passed, 10 skipped, 8 warnings, 55 error in 88.36 seconds
after:  466 failed, 253 passed, 12 skipped, 8 warnings, 49 error in 90.15 seconds