In what ways does Conrad reflect the British attitude of Orientalism toward Africa in "Heart of Darkness," and in what ways does he challenge such attitudes? Explore through Edward Said's framework of Orientalism.

Orientalism refers to the tendency of the West to romanticize Islamic or Middle Eastern cultures. According to Edward W. Said in his book "Orientalism," Western powers tend to fetishize Middle Eastern cultures as backward, exotic, and hedonistic. Orientalism characterizes the Occidentals (Westerners) as civilized peoples and the Orientals (Easterners) as uncivilized barbarians.
Edward Said maintains that the Western tendency to infantilize Middle Eastern peoples is just one way to justify Western imperialist and hegemonic ambitions. Additionally, the literary portrayal of the West as masculine/strong and the East as feminine/weak validates the Darwinian view of Easterners as culturally and biologically inferior. The fictionalized and simplistic characterization of Easterners as degenerate heathens in need of saving reinforces only the Western conviction at the expense of Eastern reality. Having said this, I would argue that the text and narrative of The Heart of Darkness betrays Conrad's typical British prejudice as well as his sympathy towards the native peoples.
Much like his peers, Conrad portrays the natives as savages and cannibals incapable of civilized conduct. In the novel, we see Conrad's stance reproduced in Marlow's voice. When Marlow comes across a chain gang at one of his company's upriver stations, he notes the "deathlike indifference of [the] unhappy savages" who march past him. To Marlow, the scene is surreal but expected. After all, the native prisoners are "criminals" who must reap the consequences of their aberrant behavior. A little later, however, he comes across some native laborers who are obviously in the throes of death. The text tells us that he is utterly horrified by the physical suffering the dying laborers endure. Marlow realizes that he does not view the dying natives as "criminals" or "enemies." Their palpable suffering and hopeless plight inspires something akin to sympathy in him.
Later, Marlow sails upriver to Kurtz's location with the manager of the station and some "pilgrims." The twenty crewmen on Marlow's repaired steamer are called "cannibals." Here, Conrad's Orientalism betrays itself quite clearly. The natives are portrayed as servile creatures under the direction of intellectually capable westerners. Marlow indulgently calls these cannibals "fine fellows." 

They were men one could work with, and I am grateful to them. And, after all, they did not eat each other before my face: they had brought along a provision of hippo-meat which went rotten, and made the mystery of the wilderness stink in my nostrils.

As the small group of travelers sail upriver, Marlow maintains that he can sometimes hear the faint sound of drums beating in the distant forest. The sound of the drums is equated with the primitivism of the native peoples. It is this primivitism that conjures up images of violent savagery in the Western mind. Marlow's apprehension is echoed in Kurtz's call to "exterminate the brutes."

...we whites, from the point of development we had arrived at, ‘must necessarily appear to them [savages] in the nature of supernatural beings—we approach them with the might of a deity,’ and so on, and so on. ‘By the simple exercise of our will we can exert a power for good practically unbounded,’ etc., etc

Here, Conrad demonstrates the typical Orientalism of his English peers: the natives are a threat to civilization; therefore, they must be dominated, educated, and initiated into good society. Despite his biased stance, Conrad also challenged the prevailing attitudes of his time. A very good example can be seen in the passage below:

It was unearthly, and the men were—No, they were not inhuman. Well, you know, that was the worst of it—this suspicion of their not being inhuman. It would come slowly to one. They howled, and leaped, and spun, and made horrid faces; but what thrilled you was just the thought of their humanity—like yours—the thought of your remote kinship with this wild and passionate uproar. Ugly. Yes, it was ugly enough; but if you were man enough you would admit to your self that there was in you just the faintest trace of a response to the terrible frankness of that noise, a dim suspicion of there being a meaning in it which you—you so remote from the night of first ages—could comprehend."

Here, Marlow/Conrad proclaims that all humans have the capacity for primitive or raw rage. He argues that it takes courage to even admit this. Essentially, Conrad is challenging the notion of the Other (as Easterners are characterized) as intrinsically inferior or foreign to the Western nature.