Filezilla has changed one of their default settings which will destroy your forum's avatars and attachments if you depend on it to transfer your files.
There is a setting under the Edit menu --> Settings --> Transfers --> File Types: "Treat files without extension as ASCII file."

This should be unchecked when transferring directories that contain avatars or attachments because these are binary files.
kerbob has written instructions for backing up your site using Filezilla.
Note to all FTP users: If you are backing up your site via FTP and you're not changing the mode of transfer for files without extensions.. your doing it wrong and risk having bad backups. The process below is specific to FileZilla but applies to any FTP client that auto determines the transfer file type (ASCII or BIN) for certains files.
SMF BACKUP PROCEDURE USING FILEZILLA FTP
1.) Make certain that FTP client is configured to use transfer type AUTO for all files and that "Treat files without extension as ASCII file" is checked and "Treat dotfiles as ASCII files" is checked.
2.) Re-check Step 1. Highlight all remote folders on your ftp site EXCEPT the Attachments folder and download them. Why? These folders contain ASCII files without extensions and must be transferred as ASCII.
3.) Change the FileZilla configuration so that "Treat files without extension as ASCII file" is unchecked.
4.) Re-check Step 3. Highlight the remote Attachments folder and download it. Why? This folder must be downloaded as Binary because it contains Binary files (jpg, gif, png, etc).
Does anyone know if there is any other folder within SMF that may have BINary files that have no extensions?
It is also a good idea to make changes (updates, modifications, etc.) on a test forum before changing your live forum, because you have an opportunity to check that everything is working correctly before you make irreversible changes to your live forum. Using a test forum prevented me from losing my avatars and attachments because I noticed the problem on my test forum first, and left my live forum alone until I found the problem.
To make a test forum which is a mirror of your existing forum:1. Backup your current forum's files and database. If you are using Filezilla, be sure to follow kerbob's instructions above. I like to use phpMyAdmin to back up my database, but the SMF Admin section also provides a method to do this.
2. Create a new space within your website for the test forum, and upload the files you backed up. Again, if you are using Filezilla, be sure to follow kerbob's instructions above.
3. Create a new database through your web host's control panel with phpMyAdmin. Make note of the database name, username, and password which you created when you made the new database.
4. In the new database, use phpMyAdmin to restore your backed up database from the live forum.
5. Upload the file "
repair_settings.php" to the root directory for your test forum. (For example, if you put your test forum in the directory,
www.example.com/testforum/, put repair_settings.php in that directory. I would recommend using a unique name for your test forum directory, as "testforum" is too easily guessed. If you're of the tinfoil hat variety, I would also recommend password protecting the directory with .htaccess.)
6. Run repair_settings.php by going to
http://www.example.com/testforum/repair_settings.php and change all the entries to match your test forum.
What is repair_settings.php?
If you do not do this, any changes to the database, theme, posts, etc. that are done through the forum software will be made to your live forum instead and defeat the purpose of having a test forum.
7. Important: DELETE repair_settings.php from your site; it can be used to get your database password, and to change crucial settings that make the forum work.
8. Test your test forum.
Thanks to kerbob for the Backup instructions and for bringing this issue to light. There is more discussion in the thread:
http://www.simplemachines.org/community/index.php?topic=370199.0Thanks to Jason of
CharlottezWeb.com for the instructions and video tutorials (I hope he doesn't mind me linking them) and for being a super great host for SMF forums.
There is more information about restoring a MySQL database here:
Restoring a MySQL DatabaseAnd yet more about backing up and restoring here:
[Tutorial] ''How to backup & restore your forum in several different ways''