FS Game System will use the Game Boy Advance (World) BIOS if found. The BIOS is found using checksum, and file name does not matter as long as the file is indexed in the file databased.
[BIOS] Game Boy Advance (World).gba SHA-1: 300c20df6731a33952ded8c436f7f186d25d3492
Legend of Zelda, The - The Minish Cap (USA).gba (Stored in
Save.ini B4BD50E4.eep B4BD50E4.sav Mednafen/ B4BD50E4.mc0
The UUID is the variant UUID, and the base name for .sav, .eep, .state files (etc) is the first 8 hexadecimal letters of the SHA-1 ROM checksum. In this example, Mednafen has saved one save state (.mc0) in addition to the save RAM file (.srm).
Mednafen note: Mednafen sometimes seems to create both a .sav and an .eep file, the .sav being filled with zero-bytes. Probably due to not knowing up front that .eep is going to be used. Manually specifying sram_type as eeprom fixes this and causes Mednafen to skip creating the .sav file.
What’s the deal with... GBA save files? http://zork.net/~st/jottings/GBA_saves.html