The Horror of the Heights by Arthur Conan Doyle

The film looks like an adaptation of the A.C Doyle short story "The Horror of the Heights" written in 1913:

(From Wikipedia)

The story is told through a blood-stained notebook discovered on the edge of a farm in Withyham. The notebook is written by a Mr. Joyce-Armstrong, and the first two and last pages are missing; the notebook is thus dubbed the "Joyce-Armstrong Fragment".

Joyce-Armstrong, a brave aviator, had been curious over the deaths of certain aviators who tried to break the current height record of 30,000 feet. Recent casualties involve some strange deaths. One, Hay Connor, died after landing while he was still in his plane, while another, Myrtle, was discovered with his head missing. Joyce-Armstrong speculates that the answer to these deaths may be the result of what he calls "air-jungles".

Joyce-Armstrong takes his monoplane to a height of 40,000 feet and is nearly hit by three meteors. It is then that he learns that his speculations are right: entire ecosystems (air-jungles) exist high in the atmosphere, and are inhabited by huge, gelatinous, semi-solid creatures. After going through a flock of jellyfish-like and snake-like beings, Joyce-Armstrong is attacked by a more solid-looking vulture-like creature, which he narrowly escapes from. He then returns to the ground.

The aviator writes he will be going up again to the air-jungle to bring back proof of his discoveries, but here the fragment ends, save for one last sentence which reads:

"Forty-three thousand feet. I shall never see earth again. They are beneath me, three of them. God help me; it is a dreadful death to die!"

The narrative outside the notebook then explains that Joyce-Armstrong has been missing and that his monoplane was discovered in a wreck on the borders of Kent and Sussex.