Its the image displayed on the menu for the map, You know those little thumbnails, pretty sure those and .blf files. They existed in Halo 3 aswell.
C_010 is likely the map designation and the _SM tag is likely the smaller version of the image.
Its not the file from reach that is 29kb
However, C_010 is the first level of the game. (The opening cutscene actually, like arrival)
c_M10.blf would be the first actual campaign mission
M_10 would be the first multiplayer map in the list
FF_10 would be the first firefight map in the list
SO_S1E1_10 or something would be how Spartan ops content will look.
Basically if the leak is legit the guy has access to the opening movie of campaign and z00_testchamber (not the z00 tag, which imply s its not an actual content map) which would likely be the dev room which has all the textures all the weapons and vehicles and equipment laid out for testing purposes.