As an observation, the way the side panels have 'seams' folded over, i would suggest it's made of canvas rather than wood panels. Perhaps it's a mock up for training purposes?

Edit: I cannot find any evidence of tanks being used by the Bulgarian army during WWI. The uniforms do indeed match closely to WWI uniforms of Bulgaria, but the picture is difficult to make out insignia or details as to which unit. The 'tank' has what look like real artillery inside a mock-up shell of a design that is unlike any other tank I could find from the period. Granted though, tanks were new tech at the time and experimental variation was common.

This is speculation, but it's likely that if they are indeed Bulgarian, this was intended to train either artillery or engineering units on how to disable enemy tanks, or, how to operate a captured tank.

Is anyone aware of strategies used by other WWI armies to familiarise their troops with these 'new' machines?