If I had to blindly guess it's the same long standing problem of the value from the default attribute on a form field definition being used when a field is submitted with an empty value.

Michael is correct with the reason.

However I wouldnt try to "fix it" because the reason for this is so that you can reset the value of a field to the defaults.

Its also reasonable to assume an admin might not want to block any MIME Types, and therefore the catch 22 exists - you can't allow all because doing so resets the default.

and if the underlying reason for this is as stated - then this has knock on effects for ALL fields where an admin might rightly want to remove the default and save a blank value.

I doubt that there is a real world example where a default is present and you would want to make it blank. I'm not going to take the time to try and find one.

I already gave one.

delete the value text/html from illegal mime types (which is perfectly valid to do, a admin might want a user to be able to upload html files)

ok ouch i withdraw my earlier comment

no one else complained in the last decade so can't be that important.

Is that what is causing #38235 ?

Is that what is causing #38235 ?

I think that's the case.