I struggled a bit with a strange "File upload failed!" error; I finally found the cause to be the following line in program/steps/mail/attachments.inc:
$attachment = $RCMAIL->plugins->exec_hook('upload_attachment', $attachment);
According to the code I had in plugins/filesystem_attachments/filesystem_attachments.php, that line should be:
$attachment = $RCMAIL->plugins->exec_hook('attachment_upload', $attachment);
...which fixed my problem.
I double-checked my file-modification dates and it doesn't look like I had a mix of old and new files, and I did an svn update while trying to figure this one out. I find it hard to believe that this bug would be in the svn trunk; surely someone else would've run into it by now.
Anyway, there it is, just in case.
- R.