16. The non-transitory computer-readable storage medium of claim 14 , wherein the predefined sentence level features comprise at least one of an anchor feature, a noise feature, a length feature, a thread level feature, an anaphora detection feature, or a lexicon feature.
14. A non-transitory computer-readable storage medium having computer-executable instructions, the computer-executable instructions when installed onto a computing device enable the computing device to perform actions, comprising: receiving a message having at least one attachment; using a machine learning model to select a set of sentences from within the message to be relevant to the at least one attachment based on a set of predefined sentence level features; identifying from within the set of relevant sentences a set of keywords further relevant to the at least one attachment; and associating at least one of the keywords in the set to the at least one attachment, such that the association is useable for at least one of indexing or searching of the at least one attachment.