Validating svg file
In case you have more files, or want to trigger the check upon changing the file and not only in form submission, use such code instead: I'd just like to add that using "on Submit" instead of "on Change" is cumbersome - especially if the "multiple" option is used.
There's no error checking on these to keep them short, presumably if you use them you'll make sure the input exists first and the extensions array is valid!
The organization tries to foster compatibility and agreement among industry members in the adoption of new standards defined by the W3C.
Incompatible versions of HTML are offered by different vendors, causing inconsistency in how web pages are displayed.
But see the other answer that adds validation to the field after the file has been selected - stackoverflow.com/a/8492937/1754743 As stated above, it's not possible out of the box.
The simpler solution that I've found: use a Regular Expression Validator to check the file extension. Of course, it only checks the extension, not the file content (you must use server-side code and inspect the bytes), and does not change anything to the file list displayed in the folder browser.