Fix inline images for eml exports (#4879)

We have to use `contentId` instead of name for inline attachments as filenames can be duplicated.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

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- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

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* #<issue>

#### Test Plan

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- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/converters/eml/eml.go

@@ -2,8 +2,12 @@ package eml
 
 // This package helps convert from the json response
 // received from Graph API to .eml format (rfc0822).
-// Ref: https://www.ietf.org/rfc/rfc0822.txt
-// Ref: https://datatracker.ietf.org/doc/html/rfc5322
+
+// RFC
+// Original: https://www.ietf.org/rfc/rfc0822.txt
+// New: https://datatracker.ietf.org/doc/html/rfc5322
+// Extension for MIME: https://www.ietf.org/rfc/rfc1521.txt
+
 // Data missing from backup:
 // SetReturnPath SetPriority SetListUnsubscribe SetDkim
 // AddAlternative SetDSN (and any other X-MS specific headers)
@@ -17,6 +21,7 @@ import (
 	mail "github.com/xhit/go-simple-mail/v2"
 
 	"github.com/alcionai/corso/src/internal/common/ptr"
+	"github.com/alcionai/corso/src/internal/common/str"
 	"github.com/alcionai/corso/src/pkg/logger"
 	"github.com/alcionai/corso/src/pkg/services/m365/api"
 )
@@ -152,8 +157,23 @@ func FromJSON(ctx context.Context, body []byte) (string, error) {
 				return "", clues.WrapWC(ctx, err, "invalid content bytes")
 			}
 
+			name := ptr.Val(attachment.GetName())
+
+			contentID, err := attachment.GetBackingStore().Get("contentId")
+			if err != nil {
+				return "", clues.WrapWC(ctx, err, "getting content id for attachment")
+			}
+
+			if contentID != nil {
+				cids, _ := str.AnyToString(contentID)
+				if len(cids) > 0 {
+					name = cids
+				}
+			}
+
 			email.Attach(&mail.File{
-				Name:     ptr.Val(attachment.GetName()),
+				// cannot use filename as inline attachment will not get mapped properly
+				Name:     name,
 				MimeType: kind,
 				Data:     bts,
 				Inline:   ptr.Val(attachment.GetIsInline()),