Resolve "Restore file drag & drop"

What does this MR do (summary)?

Closes #71 (closed) Files can now be dragged and dropped on the submit dialog

Screenshots

Without a file being dropped:

File dropped with different themes:

What changes have you made (detailed)?

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.