Fix downloading binary images to device

[penn5]

BytesIO is used if no length is provided, so the correct type here is bytes. Also binary images are often flashed, and especially the Android bootimage header contains 0x80 which cannot be decoded in the default codec for 'r' type, which is default.

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[coveralls]

Coverage remained the same at 43.567% when pulling 633fe32 on penn5:patch-1 into d9b94b2 on google:master.

[penn5]

I signed it.

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