Some cape textures are (correctly) returned with
Content-Type: image/png
while others are (wrongly) returned with
Content-Type: application/octet-stream
This does not seem to affect skin textures.
Some examples:
3f1ba7ee6f697c46f1dfc963687c3ca29a7fe52d9fe652592e6730e0ba4d application/octet-stream
a7893270f7df67a20ea9c8ab52d7aff23270fc43a11fb3dae62291634af424 application/octet-stream
3f688e0e699b3d9fe448b5bb50a3a288f9c589762b3dae8308842122dcb81 image/png
95a2d2d94942966f743b84e4c262631978253979db673c2fbcc27dc3d2dcc7a7 image/png
I’m guessing Mojang uses
application/octet-stream to make the file auto download but this is not the correct way to do this with headers. Here are some solutions: https://stackoverflow.com/questions/20508788/do-i-need-content-type-application-octet-stream-for-file-download