Rename your game.gb file too before uploading to Itch, helps when downloading and saving to a flash cart with other same named ROMs. Dashosaur_DX.gb or similar 👌🏻
I guess you picked the ROM file from the web or windows build, right?
Because indeed in those builds, the rom file is named "game.gb" because of the web wrapper it is included in.
Otherwise if you look at the downloadable files at the bottom of this page, above the comments section, you will see that there is a rom file named "dashosaur-dx-v021.gb" available to download so that you don't need to rename it yourself.
Maybe I can have a look how I can modifiy the web wrapper to launch a renamed rom file. Thanks for the info, and hope you have fun.