The problem is to convert .msg to .eml, a workaround is to use a library that does it directly like this one then send the converted .eml.

This might be the easiest fix. If you want to still load a .msg and build an .eml file from it in your custom code, you have to fix your code for including correctly attachments and linking them correctly in the html (there's missing fields,...)

Note : .msg files have a size limit of 14MB so .eml could be better to use instead anyways.