The mc_issue_attachment_add method declares it wants the attachment encoded in with the base64 method:

<code>
Input:
use: encoded
namespace: http://futureware.biz/mantisconnect
encodingStyle: http://schemas.xmlsoap.org/soap/encoding/
message: mc_issue_attachment_addRequest
parts:
username: xsd:string
password: xsd:string
issue_id: xsd:integer
name: xsd:string
file_type: xsd:string
content: xsd:base64Binary
</code>

but no decoding is made before storing the actual file so it looks like "corrupted" when getting it back.