Conditional required based on if an image or file is attached.

[DanielKurmann]

Hi

I am looking for an image question to be required if a file is not attached and vice versa. 

I am aware of the string-length function which can be used to hide a question based on if an image is taken. Is there an equivalent to this for the required column. 

Thank you.

[ChristopherCounsell]

${your_photo_question}!=''