Way to check file at upload transaction

**Is your feature request related to a problem? Please describe.**
I want to have a way to check if the file i added as an attachment is indeed the file i tought. (Make it downloadable again)

**Describe the solution you'd like**
Make it so i can also dowload it when i am still creating the transaction

No files, tests, or entry points are named. Start by tracing how attachments are uploaded during transaction creation and whether a download path already exists. Done means a user can verify the selected attachment by downloading it before completing the transaction.

Written by the indexing model from the issue text.