Step 1: Go to HTTP Trigger and create an object.
Step 2: Go to Event Registry to register it and then Activate the trigger.
The particular URL that will be used is (here broken up into different parts):
The Data file should be posted as >> Content-Type "multipart/form-data"
The Activity ID is the entity ID of the HTTP Trigger (to get this go to object View and copy paste the entity ID here).
The Group ID is also the entity ID of the group where the process flow is created.
In the Process flow, we need to replace the existing Source object with Context Source. And double click on the Context Source object, go to Properties panel and make the eventContextEnabled property to True.
Keep the rest of the flow as is.
Once the file is posted to the URL, it will automatically trigger the transaction and the Data file will be treated as the source file.