Bug 1074585 -  TEST-UNEXPECTED-FAIL | /builds/slave/test/build/tests/xpcshell/tests/mailnews/compose/test/unit/test_detectAttachmentCharset.js | "Shift_JIS" == "UTF-8. r=jcranmer, a=jcranmer

If we can't detect the charset, see if it is UTF-8 compatible. Use UTF-8 if so.
If not, don't set a charset.
Bug 1071069 - Thunderbird PFS removal to fix MozMill tests: Small comment fixes. r=mkmelin a=comment-only-fix