Not feeling 100% about my Controller design.
The logic for reading the Excel file (i.e. everything from var connectionString until before the return) belongs in a model method, not the controller.

You might also want to handle the case when the uploaded file isn't an Excel file or it doesn't have the columns you expect it to. Depending on how you use this or who uses it, this might not be necessary, but if normal users are allowed to upload files, they might very well upload the wrong file even if you tell them not to. And in that case a meaningful error message leads to a better user experience.

Regarding the question in your comment: As I said, I don't know how this is going to be used, but I can't think of a situation where it would be helpful for the user to be able to re-upload the file by hitting the refresh button. So yes, redirecting to an UploadComplete action makes sense.