@danielflc
When using the full post editor (not the one at the bottom of topics, click the preview button to see it) there's some tabs below the message area. One of them says attachments. Click that to upload files.
Here's what the tabs look like:
2020-07-26_065527.png
Click the attachment tab to display the area for uploading attachments:
2020-07-26_065532.png
Attachments can be left attached to the post (displayed at the bottom in a group) like so:
2020-07-26_065929.png
or be placed inline to have them appear somewhere in your actual message. Useful for things like pictures or tutorials with files.
2020-07-26_070222.png
The placement of inline attachments is dependent on where your cursor is inside the message area but can also be moved around by cutting/pasting the bbcode the place inline button generates. It looks like this:
[attachment=0]2020-07-26_070222.png[/attachment]