Supported mime-types for attachments:

text/plain
text/html
text/pdf
text/x-pdf
multipart/alternative
application/pdf
application/x-pdf
image/png
image/jpeg
application/excel
application/x-excel
application/msword
application/vnd.openxmlformats-officedocument.wordprocessingml.document
application/vnd.openxmlformats-officedocument.spreadsheetml.sheet