What kind of file is it if its not one of the following:
JPEG, GIF, PNG, APNG, TIFF, BMP, PDF, XCF (GIMP). Please note that the TIFF, BMP, PDF and XCF formats will be converted to PNG on upload.
Just save the sprite as a png with the gamemaker sprite editor and upload.
Actually i changed it to PNG and uploaded it but the file was corrupted.
But it's ok i found a good website with really good tilesets so I'll just go and start from scratch...again.