A trigger would not be the right terminology or method of doign this. Seems to me the best solution here is a DTS package that when completed, gets scheduled as a job that runs every X minutes.
One, borrowing on what RossC said, make sure that you have a way/place to move a file once it is processed to avoid reprocessing.
Lets assume your directory structure is like this
Create a new DTS package
Add some a package level Global Variable and lets call it targetFile
Make a data import task that will import an excel file (assuming they will always use the same formatting
Name that task something easy to remember, and make it get the name of file it needs to import from the DTSpackage Global Variable object named targetFile
Add an ActiveX script task, and use something similiar to the following to set the global variable
Set FSO = CreateObject("Scripting.FileSystemObject")
Set OFiles = FSO.GetFolder("D:\Uploads\ForProcessing")
For Each iFile In OFiles.Files ' For each file that is in the target Directory
' Set the Value of the Global variable here
DTSGlobalVariables("targetFile").Value = iFile.ParentFolder & "\" & iFile.Name
'Look up some code that will execute your import task
' EXECUTE YOUR DATA IMPORT TASK HERE
' Move the targetFile to the other direcotry to avoid reprocessing
iFile.Move("D:\Uploads\ForProcessing\" & iFile.Name, "D:\Uploads\Processed\" & iFile.Name )
Set OFiles = Nothing
Set FSO = Nothing
Main = DTSTaskExecResult_Success
Fill in the blanks where needed to suit your needs and you should be pretty good to go.