I've created a custom document type, and exported the UTI as well as setting the document info in my plist. I've also created a preview extension for the document, which displays properly. If I select the 'Copy to myApp' action on the action sheet, it correctly opens my application and I'm able to see the expected results, so long as I'm not using iMessages to preview the document. I've tested from the Mail app, and from the Files app successfully. When performing the action from iMessages however, my app stays in the background and does not recieve any messages (that I can determine) nor does it gain focus.
I've currently set the Document Types, Exported Type UTIs, Supports Opening Documents In Place, and Application Supports iTunes File Sharing keys in the plist.
Any pointers to documentation or help would be greatly appreciated as I'm not quite sure where to even begin troubleshooting this. Given that it works in other applications, but not from iMessages, I'm confident my exported UTI and document consumption is correct.