I have been looking around trying to find a ruling on what happens when attachments are stacked. If you stack an attachment that gives bonuses, do you only use the highest bonus or do they add? For example: Aegon's Blade and Blood of the Dragon? If the character has a military icon does he get +8 str in a challenge or just the highest +6? I am sure it is out there, but I haven't had luck in finding it. Thanks in advance.

What you're looking for is in the FAQ in the "Timing Structure and Flowcharts" section:

"Lasting Effects Conflict:

Even if not triggered at the same time, multiple Lasting Effects may affect the same card at the same time. For example: a character without the Power icon is affected by two lasting effects. One (such as Slander) removes a Power icon from the character, and one (such as Noble Lineage) adds the Power icon to that character. In this example the two lasting effects cancel each other, and the character is left unchanged. The order in which the lasting effects take place is irrelevant: the net sum result of all lasting effects is applied to the character (the net sum of -1 Power icon and +1 Power icon is 0, thus the character remains unchanged). A character can never have more than one icon of a specified kind.

Lasting effects that affect other character attributes work in the same fashion. For example, if a character has a STR of 2, and a lasting effect (such as Dark Wings, Dark Words) lowers that character's STR by 3, and another Lasting Effect (such as Hear Me Roar) gives the character +2 STR, the net sum modifier affecting the character is -1 STR (-3 +2 = -1). Thus, the character has a total STR of 1. If a character's STR is ever lower than 0 after all effects are applied, its STR is rounded up to 0."

So the answer to your question is that the multiple STR bonuses, whether from attachments or anything else, are added together and the result applied to the character.