Search by file extension

When querying the REST API search, is there any way to restrict the search to a given file extension? I'm attempting simply /objects?o=0&q=pdf to find all pdf files but get an error from the API

{
    "ErrorCode": "159187064",
    "Status": 500,
    "URL": "/objects",
    "Method": "GET",
    "Exception": {
        "Name": "COMException",
        "Message": "Your search request yielded too many results. Use more precise words in your search request.",
        "InnerException": {
            "Name": "MFilesException",
            "Message": "Your search request yielded too many results. Use more precise words in your search request.",
            "StackText": "Error reference ID: 37c9e69e-d8f0-4998-97f6-4c037532c5d0",
            "ErrorCode": "159187064"
        }
    },
    "Stack": "Error reference ID: 37c9e69e-d8f0-4998-97f6-4c037532c5d0",
    "Message": "Your search request yielded too many results. Use more precise words in your search request.",
    "IsLoggedToVault": true,
    "IsLoggedToApplication": true,
    "ExceptionName": "COMException"
}

Which is a little funny because leaving off the q parameter entirely and simply getting all documents returns everything without issue.

I attempted to find a property that might reflect the file extension, hoping to query that way but couldn't find one that matched.