The files' names are quite long to help you find the right icon for your aircraft.
However, for Taranis and Horus (OpenTx) owner, rename the icons with a 6 characters max file name (ie: 'ext300.png')
For Taranis owners, file's extention is BMP (ie: 'ext300.bmp')
For Horus (OpenTx) owners, file's extension is PNG (ie: 'ext300.png')
For Horus (FrOs) owners, file's extension is JPG (ie: 'ext300.jpg')