Facebook's inApp browser crashes when trying to upload photos in a form

I've got a registration form on a website that requires users to upload photos of themselves. We're sending the link to this form on Facebook, and most of our users open it on mobile, via the inApp web browser of the Facebook App.

I tried to debug the problem with several devices and operating systems:

iOS 7&8: When clicking on the input field, the dialog to use camera or existing images shows up. Whatever I click, the InApp browser crashes, and I can't visit the link until the FB app is restarted.

Android 4.3: The InApp browser doesn't crash, but no dialog box comes up whatsoever. So when the user click on the file upload field, nothing happens.

I tried to insert a script which detects if the form is opened by the Facebook InApp browser and redirect to a _blank page (for it to open in the browser), but that doesn't work either, and I'm out of options.

It's not really an option to output a message to the user that he/she has to open the form in a browser.

Is there an easy way to fix this apart from these?

Edit: The problem doesn't occur in the FB Messenger app, only with the normal FB app.