Handle duplicate file uploads.

[uawasth2]

update frontend and backend code to prevent files with duplicate names from being uploaded.

[mattwalo32]

Instead of immediately adding the filename, we could submit the post request to upload the file, then check if there's already any other existing files with the same name. Then add _1 or _2, etc to the end of it to prevent duplicate file names. The frontend can use the response of the endpoint to get the name of the file. I.e. the backend has final say over file names.