Force Bamboo.email to create email not using "multipart/alternative" mime-type

By default, Bamboo.Email, when you only add a text_body/2, appears to create emails with the following structure:

Content-Type: multipart/mixed; boundary="----=_Part_301556352_2177739793.3928492858"

------=_Part_301556352_2177739793.3928492858
Content-Type: multipart/alternative; boundary="----=_Part_578078571_1285899129.711799163"

------=_Part_578078571_1285899129.711799163
Content-Type: text/plain;charset=UTF-8

Text body here
------=_Part_578078571_1285899129.711799163--
------=_Part_301556352_2177739793.3928492858--

While this is a good general-purpose structure that allows text and HTML bodies along with attachments, and is perfectly valid, the presence of a text/plain section without a text/html section triggers some spam filters.

Is there a way to tell Bamboo.Email to create an email that doesn’t use multipart/alternative ?