Only decode attachments once when using the SOAP API

Attachments uploaded using the SOAP API calls are decoded by NuSOAP. Up till
now, we are also decoding the attachments, which caused the attached files
to be unreadable.

Although workarounds do exist, the correct way is to let NuSOAP apply the
Base64-decoding. The requirement for the users of the SOAP API is to
Base64-encode the attachments passed to mc_issue_attachment_add
mc_project_attachment_add and to Base64-decode the attachments retrieved
with mc_issue_attachment_get and mc_project_attachment_get.

Fixes 0011151: Can't upload attachment to issue via soap interface