msgconv: don't set EncryptedFile if there isn't one

FIXME: This probably isn't proper handling, but this is enough to
prevent panics.

pkg/attachment/attachment.go

@@ -34,5 +34,6 @@ type ReuploadedAttachment struct {
 	AttachmentReupload
 	DownloadedSize int
 	MXC            id.ContentURIString
-	EncryptedFile  *event.EncryptedFileInfo
+	// This can be nil if the room isn't encrypted.
+	EncryptedFile *event.EncryptedFileInfo
 }

pkg/msgconv/from-discord.go

@@ -488,9 +488,11 @@ func (mc *MessageConverter) renderDiscordLinkEmbedImage(ctx context.Context, int
 	}
 	preview.ImageSize = event.IntOrString(reupload.DownloadedSize)
 	preview.ImageType = reupload.MimeType
-	preview.ImageEncryption = &event.EncryptedFileInfo{
-		EncryptedFile: reupload.EncryptedFile.EncryptedFile,
-		URL:           reupload.MXC,
+	if reupload.EncryptedFile != nil {
+		preview.ImageEncryption = &event.EncryptedFileInfo{
+			EncryptedFile: reupload.EncryptedFile.EncryptedFile,
+			URL:           reupload.MXC,
+		}
 	}
 }