Resolution:
BC is treating the inbound payload as binary because the "Content-type" in the MIME header is set to "application/octet stream" as shown below:

------=_Part_265_1758317315.1315550611433
Content-Type: application/octet-stream; name="N/A"
Description:
============
When receiving an inbound AS2 payload with EZComm, how does BusinessConnect determine whether the payload is text or binary?

Environment
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TIBCO BusinessConnect 5.3.X

Resolution:
=========
BusinessConnect (BC) examines the AS2 header in the inbound payload.  If the payload has the  "Content-type" in the MIME header set to "application/octet stream" as shown below:

------=_Part_265_1758317315.1315550611433
Content-Type: application/octet-stream; name="N/A"
Content-Transfer-Encoding: binary
Content-Disposition: attachment; filename="N/A"

BC will treat the payload as binary.

For BC to treat the inbound payload as text, have your trading partner change the "Content-Type" to "text" (i.e. "text/plain", "text/xml").