fix: inner mime type quoted-printable

[mgoeppe]

• multipart.Reader.NextPart() hides the Content-Transfer encoding. Therefor I switched to NextRawPart() and needed to add the decoding to the decodeContent(..) func.
• I also could have changed the detection of embedded files, sticked to NextPart() and used the mime decoding of multipart itself. I decided against that approach in order to stick close to your implementation.