The document created from a mail with an inline image will not retrieve the image and therefore will not display it in the Text field.
The mechanism implemented in the class ExtractMessageInformationAction to extract the mail information ignores some mail Parts if the mail MIME type is multipart/alternative. And attachments are only read if the MIME type is multipart/*
Once the attachment will be correctly retrieved, it will be necessary to replace a part of the mail body to make a reference to the document files
Please find attached an example of message with inline attachment.