Use file extensions

[Kruemmelspalter]

Xournal++ Editor automatically saves with .xopp file extension

Fix Options

• make the client move the ${ID}.xopp file to the origin file ${ID} -> Problem: Editor needs to be closed for it to update, Only temporary but easy
• make the client have a file watcher and update the document file whenever the xopp file is updated
• make the backend able to handle file extensions

[Kruemmelspalter]

Frontend or Backend could use the upload file name for automatically getting the extension