In some cases, the file can be blocked by URL or other characteristics without transferring the entire file.
This may be why you are seeing "prevent" in some cases and "inspect" in others.
It's likely a side effect of leaving Resource Classification in Background.
If you're concerned about end users being impacted by putting Resource Classification in Hold, create a separate Threat Prevention profile just for your mail server where it set to Hold.
Create a Threat Prevention rule that refers to your email server in the Protected Scope and apply your new profile.
Mail is pretty fault-tolerant and should not be negatively impacted by this.
In the Threat Prevention profile with regard to extensions
- Inspect means submit to ThreatCloud for analysis and allow only if deemed safe.
- Block means don't allow the attachment at all (irrespective of the file