Commented Unassigned: emails with excel files get filtered out EPPlus 4.0.4 [15278]

recently upgraded from 3.1.3.3 to 4.0.4. yesterday got a complaint from a client that emails with excel files attached, created by epplus were getting filtered out and removed in messages sent externally. the excel file was replaced with a text file, with the same name filename.xlsx.txt with one line "This attachment was removed.". doing some googling i found out that the exchange edge server was doing the filtering. I told my sys admin and he tried to ensure that xlsx files would not get filtered out by the edge server. he tried a bunch of settings and couldn't get it to work. finally this morning i remembered I had updated epplus to 4.0.4, so i reverted back to the old 3.1.3.3 and tried it and the excel file sends normally when going external.
So there must have been a change in the file signature in 4.0.4 that the edge server picks up as a virus and filters it out.
Comments: tested and problem still exists in changeset 65115680ca04