There is only one file type that uses the .OLK15MSGATTACH extension:
Outlook 2016 Email Message Attachment File
An OLK15MSGATTACH file is a data file created by Microsoft Outlook 2016, an email and calendar program. It contains information about attachments to emails sent and received by a user in Outlook. OLK15MSGATTACH files are similar to .OLK14MSGATTACH files, which are created by Outlook 2011.
When viewing an email message in the 2016 version of Outlook for macOS, the data in the OLK15MSGATTACH file is loaded. The file allows the software to operate more efficiently by loading the information only when the user needs it.
OLK15MSGATTACH files are stored in the following directory: [user]/Documents/Microsoft User Data/Office 2011 Identities/Main Identity/Data Records/Message Attachments/
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Updated: March 30, 2018