eog_thumb_data_new 

        if (result != GNOME_VFS_OK) {
                set_vfs_error (error, result);
        }

        /* check required info fields */
        if (((info->valid_fields & GNOME_VFS_FILE_INFO_FIELDS_MTIME) == 0) ||
            ((info->valid_fields & GNOME_VFS_FILE_INFO_FIELDS_MIME_TYPE) == 0))
        {
                set_thumb_error (error, EOG_THUMB_ERROR_GENERIC, "MTime or mime type not available");
        }

is missing an else before the second if. No need to set the error twice.