add caption in file attachments

[sojharo]

This is client side work for task opened on server by sumaira: Cloudkibo/CloudKibo#467

Each of our attachment should also have the label. Just like whatsapp. The iOS task for this is Cloudkibo/iOS#308

[Assadbintahir]

I have worked on it today. I have made the UI fragment which will hold the selected file before it is uploaded to server. On this UI, there will be the option to add the caption. Then the file and caption will be sent to server.

I will complete the logic work for this by tomorrow.

[Assadbintahir]

Completed work on this
I have also opened the pull request for it.