Clear attachments on reselection

- as we only show the FIRST attachment per message, we also should only send one attachment
(not sure about sending, but code looks as if all attachments gets sent)
- changing for example a video to an image or an audio was also not working, as the first attachment persists
- avoid sending wrong attachments. reproduce like this:

Add text: "He mom thats me on holiday"
Add attachment: "Me_on_Holly.jpg"
*OMG, wrong attachment. Better change...* (ignoring the delete button)
Add attachment: "Me_on_holiday.jpg"
*puh, that was close!*
send
😳*OMG²* 😲 

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I would not implement a "Do you want to change the attachment" button, to keep the UX. Also **I** never felt that more than one attachment is going out at once.

Works with drafts.
Tested KK, but should also work on GB...