MM-38702: Do not fail during importing without attachment (#20800)

We just log a warning instead of failing completely.

https://mattermost.atlassian.net/browse/MM-38702

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/file.go

@@ -476,7 +476,7 @@ func (a *App) UploadFile(c request.CTX, data []byte, channelID string, filename
 	return info, nil
 }
 
-func (a *App) DoUploadFile(c *request.Context, now time.Time, rawTeamId string, rawChannelId string, rawUserId string, rawFilename string, data []byte) (*model.FileInfo, *model.AppError) {
+func (a *App) DoUploadFile(c request.CTX, now time.Time, rawTeamId string, rawChannelId string, rawUserId string, rawFilename string, data []byte) (*model.FileInfo, *model.AppError) {
 	info, _, err := a.DoUploadFileExpectModification(c, now, rawTeamId, rawChannelId, rawUserId, rawFilename, data)
 	return info, err
 }