Fix #3983: Uploads: NoMethodError - undefined method `>=' for nil:NilClass

* Rename Upload#download_for_upload to #get_file_for_upload.

* Fix #get_file_for_upload to raise error if no file or source url was given.

* Fix javascript upload validation to disallow submitting form if file is
  not present and the source is not an url.

app/logical/upload_service.rb

@@ -48,13 +48,8 @@ class UploadService
 
       @upload.update(status: "processing")
 
-      if @upload.file.nil? && @upload.source_url.present?
-        @upload.file = Utils.download_for_upload(@upload)
-      end
-
-      if @upload.file.present?
-        Utils.process_file(upload, @upload.file)
-      end
+      @upload.file = Utils.get_file_for_upload(@upload, file: @upload.file)
+      Utils.process_file(upload, @upload.file)
 
       @upload.save!
       @post = create_post_from_upload(@upload)

app/logical/upload_service/preprocessor.rb

@@ -91,12 +91,7 @@ class UploadService
       begin
         upload.update(status: "preprocessing")
 
-        if params[:file].present?
-          file = params[:file]
-        elsif upload.source_url.present?
-          file = Utils.download_for_upload(upload)
-        end
-
+        file = Utils.get_file_for_upload(upload, file: params[:file])
         Utils.process_file(upload, file, original_post_id: original_post_id)
 
         upload.rating = params[:rating]

app/logical/upload_service/utils.rb

@@ -206,7 +206,10 @@ class UploadService
       tags.join(" ")
     end
 
-    def download_for_upload(upload)
+    def get_file_for_upload(upload, file: nil)
+      return file if file.present?
+      raise RuntimeError, "No file or source URL provided" if upload.source_url.blank?
+
       attempts = 0
 
       begin