Instead of gedit, the LibreOffice text content is not plain text. This could be one of the reasons that it doesn't allow you drag and drop its content to your input box.
One interesting fact is that if you drag and drop a content from a webpage to you document on Writer you'll get the HTML content of the page (all the tags), but if you do the same in Google Chrome, you will get the rich content (like colour, formatting, and so on). This was reported by other folks.
If you try to drag LibreOffice's content to any area on Firefox, it will be same issue.
So, the problem is not in your code, but in the way that LibreOffice (Writer, Impress, and all the other apps) integrate with the browser libraries.
Hope that help you, it's not an usual issue.
Thanks