Issue: File Uploads Fail on Some Servers

When PHP uses the default value for the upload_tmp_dir config value, VaultWiki is unable to successfully upload attachments. This issue results in data loss when importing from another wiki.

This issue should also exist in vanilla vBulletin, but it is fixed for VaultWiki attachment-type uploads (not comment uploads) in the next release by checking also the default system tmp directory, and falling back to a fake temp directory created by VaultWiki.