Cannot store email message content with attachment and image in signature

As discussed on mailing list https://markmail.org/message/ibiajgahexqqd6im,

When using "getMessageBody" method of "MimeMessageWrapper.java" to get the
email message.

It works perfectly fine for the following cases:

1. When there is a basic message with a basic signature (no images or
attachments)
2. When there is a basic message with a signature containing an image.
3. When there is a message having attachments with basic signature.

Issue:

– When there is a message with the attachment and the signature having an
image.
In the above case, all the data is stored in the database except the
actual message content (Eg.  Hello! How are you?).

Findings:

I checked the structure of the email received in all cases and find out
following:

1. In the cases where everything worked fine, the email parts identifiers
i.e ID was of "0.x" series such as "0.0", "0.1".
2. In the issue case, the email parts ID was of "0.0.x" series such as
"0.0.0", "0.0.1".

I then checked the code and found out that the implementation is hardcoded
to work till "0.x" series only. Therefore all the content which was up to
"0.x" part got stored while the actual message which was in "0.0.x" part
was not stored.

We need to make the logic more scalable.