No attaching suspect files to the forum is a no, no, as should it be detected it would be blocking the topic and possibly worse the forum.
That is why there is a restriction on what files can be attached.
You can send the sample to avast as undetected malware. From the avastUI > Support > scroll down to the ‘Report files to the avast! virus lab’ Submit to virus lab… button, use the next window to navigate to the suspect file.