Download .msg file in Appian

It depends if you need to generate the EML from the mails that Appian receives or sends.

Emails sent from Appian: you can collect all the attributes (to, cc, bcc, from, attachments...) and create the EML using this plugin https://community.appian.com/b/appmarket/posts/create-read-email-files-utilities

Emails received in Appian: The are several plugins to collect the mails from a mailbox. Most of them automatically generates the EML in a Appian folder

Depends on your requirement.

This plugin can ingest msg files but I'm not sure it can deal wtih attachments.

https://community.appian.com/b/appmarket/posts/document-indexing-services

Otherwise, if you are trying to read a e-mail in a mailbox (i.e. M365), you can use a plugin like this: community.appian.com/.../msgraph-email-poller

If you mean a .msg file attached to an email received by a process model you can download it as attachment with stripthml(msg!attachment) if you send this email from outlook web (not from the application). If you want to download this .msg file inside appian there's a plug in called "msg file reader"