@jordeliason, Here's a quick explanation from the AMP KB.
The MX Security Appliance will block HTTP-based file downloads based on the disposition received from the AMP cloud. If the MX receives a disposition of malicious for the file download, it will be blocked. If the MX receives a disposition of clean or unknown, the file download will be allowed to complete.
The supported file types for inspection are:
MS OLE2 (.doc, .xls, .ppt) MS Cabinet (Microsoft compression type) MS EXE ELF (Linux executable) Mach-O/Unibin (OSX executable) Java (class/bytecode, jar, serialization) PDF ZIP (regular and spanned)* EICAR (standardized test file) SWF (shockwave flash 6, 13, and uncompressed)
* This includes the inspection of XML-based Microsoft Office file types (.docx, .xlsx, etc...).
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