You add files to an ACF File Upload or Gallery field, save the article, and the field comes back empty. No error message, nothing in the logs, no clue about what went wrong. If your site runs Cache Cleaner (or any other tool that empties Joomla's temp folder), that's almost certainly what's deleting your files.
When you drop a file into the field, it isn't stored in its final location yet. It's staged in Joomla's temp folder, under tmp/tassos, and it stays there until you save the article, contact, or user. Only on save does ACF move it to the Upload Folder you configured.
Cache Cleaner has an Empty tmp Folder option. With that set to Yes and automatic cleaning on save enabled, it empties the temp folder at the start of the same save request, a moment before ACF gets to move your file. The file is already gone, so the field saves empty. As far as Joomla is concerned nothing failed, which is why you never see an error.
This is the fix we recommend. It leaves the rest of your Cache Cleaner configuration exactly as it is, and spares only the one folder that has to survive until you hit Save.
Log into your Joomla backend.
Go to System → Plugins.
Search for Cache Cleaner and open the System - Cache Cleaner plugin.
Open the Advanced tab.
In the Ignore Files/Folders field, add tmp/tassos on its own line.
Click Save & Close.
Paths in Ignore Files/Folders are relative to your site root. If you changed the temp folder in Global Configuration → System → Path to Temp Folder, use that folder instead, followed by /tassos.
Upload a file and save the item again. It stays this time.
If you'd rather not touch the ignore list, open the What to clean tab in the same plugin, find the Temp Folder section, and set Empty tmp Folder to No or Only via Button.
Only via Button is the plugin's own default, and it's the more sensible setting: the temp folder gets emptied when you deliberately click the Clean Cache button, not on every single save. Other extensions use that folder as working space mid-request too, so wiping it on save can bite you in places that have nothing to do with ACF.
Cache Cleaner is the tool we've seen this with most often, but the rule is general: anything that empties Joomla's temp folder while a save is in progress can remove a staged upload. That includes maintenance extensions and hosting-level cleanup cron jobs. If you run one, exclude tmp/tassos there as well.
Joomla's temp folder is a deliberate choice made by the administrator in Global Configuration → System → Path to Temp Folder. Site owners point it at a specific path on purpose: one that's writable, covered by (or deliberately excluded from) their backups, sometimes on a mounted or hardened volume.
ACF honors that setting and uses the official temp path for its temporary work, like a well behaved extension should. Writing in-progress uploads to some private location of its own would silently override a decision the administrator made, and would cause its own problems on tightly configured servers. Telling the cleaning tool to skip one folder is the safer fix, and it's a one-time change that survives future updates.
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