Wait?,There is a Flappy Bird for Fire TV?!?
Is gameplay in landscape?
The app itself has to be in landscape orientation on Fire TV,otherwise it could be simulating portrait by not using the entire screen-width,which would be as torturous as the original Flappy Bird when played on a tablet that could use landscape.
Although,portrait mode sometimes works on Fire TV for some reason.
Tip:XDA Forums supports attachments,and also supports the apk format directly.
Your limit is 20.00MB for an apk.
If it is larger than 20.00MB,try compressing it into a zip.
Zip files are also limited to a whopping 20.00MB of space.
If you have access to a Windows PC like Win7,you can take advantage of
its "Send To> Compressed (zipped) folder" a most powerful compression tool.