Hi,
During the interview, follow this logic and modify the statement to reflect the actual work - then ask the question:
OK, applicant, we will need the person we hire to have the ability to regularly lift boxes, move files, lift.. (etc..) from the floor or from a desk up to the top of a shelf or x,y,z. ( just explain what you need). The new boxes could be heavy- up to xx pounds and may or may not be broken down into smaller sizes to reduce the weight or lifting obligation.
Since this is an essential function of the position, can you tell me if this is something you could do with or without an accommodation of some sort ? If no, then no, they cant do it. If they answer yes, they will clarify one or the other:
1- yes, I can do it without an accommodation, or
2- yes, I can do it, I think, I’ll try my best…..
If #2 above, you may decide to have management / HR institute an immediate Pre-employment physical and include a lifting test- up to 50 pounds.
Best case scenario- hire the person with the biggest arms 
Also, add this lifting requirement as an essential function on the position’s job description - ASAP.
Brian Phillips