I think I've solved this myself...please feel free to comment if you see any issues:

import SwiftUI

@main

struct MyApp: App {

    var body: some Scene {

        WindowGroup {

            ContentView()

                //When the View is launched by an incoming file from another app (Mail, etc), 
                //import the file into Documents and delete the contents of Documents/Inbox

                .onOpenURL(perform: { url in
                    let FM = FileManager.default
                    let documentsdirurl = FM.urls(for: .documentDirectory, in: .userDomainMask).first!
                    let inboxdirURL = documentsdirurl.appendingPathComponent("Inbox")

                    //Call a function that moves a file to a desitination directory and increments 
                    //the name if there is an existing one with the same name
                    //the function returns the new URL

                    let fileURL = moveFile(fromURL: url, toDirURL: documentsdirurl)

                    print(fileURL.path)

                    //Cleanup the Inbox

                    do {
                        let files = try FM.contentsOfDirectory(at: inboxdirURL, 
                                                  includingPropertiesForKeys: nil, 
                                                  options: .skipsHiddenFiles)
                        for file in files {
                            try FileManager.default.removeItem(at: file)
                        }
                    }

                    catch {
                            print(error)
                    }

                })
        }

    }
}

//This function takes a file URL and and desination directory URL and moves the 
//file to the new directory it and adds a number to the end 
//of the file if there is an existing file with the same name

func moveFile(fromURL:URL, toDirURL:URL) -> URL {

    let FM = FileManager.default
    let fromFilename = fromURL.lastPathComponent
    let fromFileExt = fromURL.pathExtension
    var toURL = toDirURL.appendingPathComponent(fromFilename)
    let toFileExt = fromFileExt
    var toFilenameRoot : String!
    var toFilename : String!
    var counter = 0

    //Trim the extension from the filename

    if fromFilename.hasSuffix(fromFileExt) {
        toFilenameRoot = String(fromFilename.prefix(fromFilename.count - (fromFileExt.count+1)))
    }

    //If there is an existing file in the destination with the same name, 
    //try adding a number to then end until there is a unique name (File-1.ext, File-2.ext, etc.)

    while FM.fileExists(atPath: toURL.path) {
        counter += 1
        toFilename =  "\(toFilenameRoot!)-\(counter).\(toFileExt)"
        toURL = toDirURL.appendingPathComponent(toFilename)

    }

    //Once there is a unique filename, move the file from the source to the 
    //destination directory with the new name

    do {
        try FM.moveItem(at: fromURL, to: toURL)
            //print("file moved from:", fromURL.path, "to:", toURL.path)
    }
    catch {
            print(error)
    }

   //return the URL of the file at the new location

    return toURL
}