- 1.) I assumed you were the type who kept your promises.
- 2.) I assumed you were the type who keeps your promises.
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I prefer the second version. Using the past tense suggests that you're describing a person whose status changed: he used to keep his promises, but stopped doing so at some time. a person who keeps his promises describes someone who always has and continues to keep his promises.
However, the first version might also be used in a situation where you have a belief about the person's past status, but are unsure about whether it is continuing at the present time or will continue into the future.
In general, there's no requirement that the tense of assume must match its object. You can make an assumption in the past about the future, e.g.
Yesterday I assumed the sun will always rise.