Stricter matching for implicitly multipart filenames excludes files ending in unsupported extensions (such as foo.rhtml.bak) and without a two-part content type (such as foo.text.rhtml or foo.text.really.plain.rhtml).  Closes #2398.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@3040 5ecf4fe2-1ee6-0310-87b1-e25e094e27de