Content-disposition in eml files

Question

My java application receives eml files, opens it, detects and reads attachments.

To detect the presence of an attachment, it looks for Content-Disposition: Attachment.

However, we recived files for which the Content-Disposition attribute is missing altogether in the received eml file.

1. I was under the impression that this attribute always exists in emls. Why is this not present?
2. Is this attribute added at the source of the eml file or while transfer over network.
3. What settings need to be changed at the source of these eml files/ network transmission to include this attribute (Note: Source and method of transfer unknown)
4. What can be changed in my java application to still detect and read attachment even if this attribute is not present