I figured out why it's crashing in my code and not in the simple sample page,
If the HTML form file input has an "accept" field (e.g. "<input type="file" accept=".jpg, .jpeg" name="UploadImage" required>"), the App will crash.
Since I may not have control over the form's design, I am still looking for a solution around this issue.
You can easily test this with the code I linked in the previous post by replacing:
WebBrowser.Navigate('https://ps.uci.edu/~franklin/doc/file_upload.html');
With:
WebBrowser.LoadFromStrings(
'<HTML><HEAD><TITLE>Test</TITLE></HEAD><BODY>'+
'<form method="post" enctype="multipart/form-data" action="https://bla.com">'+
'<input type="file" accept=".jpg, .jpeg" name="UploadImage" required>'+
'<input type="submit" value="Save">'+
'</form>'+
'</BODY></HTML>','');