Style Upload picture buttons

[Sameerkash]

The < input> file tags have no styling and make it look odd compared to the rest of the app. It'd be great if they have some styling.
Although this is a bit tricky, I did it in the last sprint, so I can accomplish this with a nice-looking upload button and an image preview in the Post option.

If this sounds good, please assign this to me, I'll get the PR up soon

cc @aitikgupta , @bodhisha

[github-actions]

Thanks for raising an issue! Let us know what went wrong and we'd love to crunch it (only if it's a bug :p)

[aitikgupta]

Sounds good @Sameerkash!
(by the way, thanks for validating that our bot is working fine)