Both Exchange Server Edge role and Exchange Online Protection have an attachment filtering policy. The default in Edge Server is quite long, and the default in EOP is quite short. There is also a few values that are common to both.
So, how do you merge the lists so that your Edge Server attachment filtering policy is copied to Exchange Online in advance of changing your MX record to EOP?
Set-MalwareFilterPolicy Default -FileTypes ade,adp,cpl,app,bas,asx,bat,chm,cmd,com,crt,csh,exe,fxp,hlp,hta,inf,ins,isp,js,jse,ksh,lnk,mda,mdb,mde,mdt,mdw,mdz,msc,msi,msp,mst,ops,pcd,pif,prf,prg,ps1,ps11,ps11xml,ps1xml,ps2,ps2xml,psc1,psc2,reg,scf,scr,sct,shb,shs,url,vb,vbe,vbs,wsc,wsf,wsh,xnk,ace,ani,docm,jar -EnableFileFilter $true
This takes both the Edge Server default list and the EOP default list, minus the duplicate values and adds them to EOP. If you have a different custom list then use the following PowerShell to get your two lists and then use the above (with “Default” being the name of the policy) PowerShell to update the list in the cloud
Edge Server: Get-AttachmentFilterEntry
EOP: $variable = Get-MalwareFilterPolicy Default
There are many suggested file extensions to block in the M365 Security Center, and you can add your own extensions – for example additions in 2022 include .msixbundle and more for new executable extension types.
Recommend you review your attachment blocks and update the list to suit.