Uploaded files should prepend a space or newline before `[...](...)`

[alexmv]

Uploading a file inserts [...](...)\n\n where the cursor is -- with no proceeding whitespace. We should match the web behaviour of adding a newline before (if it's not at the start of a line already) and one one trailing newline.

CZO discussion.

[Prince2347X]

Hey, I would like to work on this issue.

From the discussion, mentioned in the issue description, the behaviour of the file uploading on the mobile app differs from the web in terms of the spacing given. Currently, in the registerUploadStart method of the compose box is simply adding two newlines i.e. \n\nafter the inline link i.e. the placeholder. So in order to fix the behaviour, the adding of two new lines should be changed to one newline at the end and one at the beginning based on the cursor position i.e. if it's on a new line or not.

The tests of the compose box will also be updated to match this change in adding prefix and suffix of new lines.

Please let me know if I should go ahead with this plan or I am missing something.