Fatal PHP error when trying to edit media gallery pages or add media

This is manifest across multiple sites on a multi-site installation, and it appears that somehow something related to the media module got hosed. When I try to upload new media to an existing gallery page it just hangs, though the media did get uploaded. If I try to add media from the library I get an "error getting media" popup. If I try to edit a gallery page I get a server 500 error. I can edit "all galleries" and I can edit media within a gallery, just cannot edit the gallery page itself or add new media to it. All existing media displays just fine.

The PHP error that pops up in the log looks like this:

[25-Nov-2012 19:07:07] PHP Fatal error: Call to undefined function file_info_file_types() in /home2/spinland/public_html/drupal7/sites/all/modules/media/media.module on line 1063

I've tried re-uploading all the modules on the server to make sure nothing got deleted, and have disabled and re-enabled Media Gallery on different sites, and nothing seems to help. I'm hoping there's some kind of configuration reset or something I can do to get this working again? I don't understand why it suddenly quit working; it was working fine for months.

If it helps, here's the cited bit of code from media.module:

function media_get_thumbnail_preview($file, $link = NULL) {
// If a file has an invalid type, allow file_view_file() to work.
if (!file_info_file_types($file->type)) { //////////////////////// line 1063
$file->type = media_get_type($file);
}

Many thanks in advance!