Why does the variable exist in the first place if you don’t want it to be stored?
As a user, I want to submit base64 encoded files using /task/.../submit-form.
And as a sad BE person, I want to capture submitted variables with a specific structure, and instead of persisting that data in the db, I want to process that variable differently - get it’s content, store as file, replace the value of that variable with a graceful solution. (And I am fully aware that even the fact of submitting the content of a file this way is BAD. But, as always, some bright decisions are left outside of my reach).
edit: btw I didn’t read through the entire thread, please disregard my comment if this has been covered
I do understand that this is becoming hard to read from the beginning. Long story short described in the previous sentences 
Can you share a test case/code that reproduces the problem?
For the test code. HM.
I think this should be in order - File upload questions - #5 by GotnOGuts
And from the BE side - try to capture it (I used @EventListener to try and replace the variable while it’s still in transaction).