The medal card for W. Jenkins would seem to show that he began his army career as a gunner in the Royal Garrison Artillery and then was transferred to the Royal Field Artillery where he served as a driver. He was entitled to the 1915 Star as well as the Victory and British War medals. However it also seems that he served in France, which he entered in 1915, no mention of Mesopotamia. He was demobbed and transferred to Z Class, the Reserves, in 1919. He may still have served in Mesopotamia, the cards are not always 100% accurate, certainly the photograph is not France! However, given that Jenkins is not an uncommon name, if I were you I'd be checking the other Jenkins medal cards, unless of course you have your man's medals and the numbers tally.
jds1949