Attachments are not deleted after uploading a new one.

[DiegoAndresCortes]

Description

If you edit a post with attachments to add more, and delete some, those 'deleted' ones aren't actually removed/deleted.

Steps to reproduce

1. Post a message or topic with some attachments
2. Edit the message and upload a new attachment
3. Delete a previously uploaded attachment
4. Save > The deleted attachment/s remain

Additional information/references

This bug only occurs on said order, it works fine if you first delete and then upload.
As a matter of fact, this bug is reproducible on the same post if you have this scenario:

Uploaded attachments

• Attachment A
• Attachment B
• Attachment C

If I upload attachment D and then delete A, and then upload attachment E and delete B and then hit SAVE:
The following attachments will be in the post:

• A
• B
• C
• D
• E

[DiegoAndresCortes]

Additionally: You can't delete attachments if you click 'delete' and cancel the first time. Subsequent attempts will not trigger the button anymore.