[CHORE] Fix swagger deprecation message

- Fix "WARNING: item list for enum is not a valid JSON array, using the
old deprecated format" messages from
https://github.com/go-swagger/go-swagger in the CI.

modules/structs/attachment.go

@@ -18,7 +18,7 @@ type Attachment struct {
 	Created     time.Time `json:"created_at"`
 	UUID        string    `json:"uuid"`
 	DownloadURL string    `json:"browser_download_url"`
-	// Enum: attachment,external
+	// enum: ["attachment", "external"]
 	Type string `json:"type"`
 }