Plugin conflict causes "upload_mimes" filter not working26 Sep 2012
Recently I need to extend some file types to be allowed to upload, a common way is to use the "add_filter" feature, but it turns out to be not working. That confuses me a lot, after googling for a while, finally got the reason.
Maybe a bug in the plugin "BackupWordpress" had conflict with the filter function. Updating the plugin to the latest version solves this problem.
It's really hard to bridge these two things together, thanks google and the guy who found that out.