Yes, this is not relocatable because the read happens inside a function. Try with the read happening at the top level into a global constant:
# Test package with sample.jpg
module MyPackage
export send_sample_file
include_dependency(joinpath(@__DIR__, "../sample.jpg"))
const SAMPLE_DATA = read(joinpath(@__DIR__, "../sample.jpg"))
function send_sample_file()
SAMPLE_DATA
end
end