Hi Jonathan. I don’t see any filters that will allow you to alter that, and you can’t use a string replacement plugin like Say What to output HTML as a replacement. The whole message from beginning to end is in one span, so you won’t be able to alter the individual pieces (like Accepted file types) separately from the actual file types.
How about hiding the Gravity Forms output altogether with this CSS:
span.gform_fileupload_rules {
display: none;
}
Then adding an HTML field right after the file upload field that contains the same information, but formatting with your own HTML markup. Would that work for you?