This change would - probably - cause updating all the translations for this message; not sure how demanding that task would be, I couldn't translate all these on my own as part of this PR for sure.

N.B.: Adding this as a draft PR since it is not complete due to this.

Regarding the message, i agree it's incorrect and shouldnt assume "uploaded files" which ultimately is a subset of any "submitted content".

See#31788 +#30931 regarding the topic of changing messages ... it's not simple indeed, so this PR may join the club :)

I tend to believe we should switch the source code to use abstract message keys (i.e.form.post_max_size_exceeded). That would overcome any future BC issues (until we want to update the actual keys, but it should be less likely).

English translations could be provided staticly, in case the Translator is unavailable, thusFormMessage::fromKey('form.post_max_size_exceeded') or so.

Instead of changing the source code, shouldn't we change the target invalidators.en.xlf?

Yes, that would fix the output and the reported problem in the easiest way.

I still think the original source/string is misleading and I would prefer to fix that in the code of the FormType class. Plus, the change invalidators.en.xlf would result in the source and target strings being different which would appear a tiny-bit inconsistent looking at the other two translations in that same xlf file.

I'm closing because:

1. we have no upgrade path to update all languages
2. the new message is technically more accurate, but that makes it more obscure to final non-techy users - so I'm not sure it's even better, from a UX perspective.

Feels free to resubmit if you have a proposal to fix those issues.
Thanks for submitting.