There are 2 file upload 'slots', one for a file and one for an image, it appears that you are trying to upload an excel spreadsheet in the image slot. There is special processing associated with the image slot, such as resizing the image as a thumbnail. Obviously you can't resize an excel spreadsheet, so the module makes sure that if you are uploading a file in the image slot that it is actually an image.

You can upload files of any type ( any type that is allowed in the Host->Settings page ), in the file slot.

You seem to be confused by the various skins (templates) that are shipped with the repository module. I would suggest you stick to the default skin until you are more familar with the module and templates ( I would also suggest reading the user/admin guide available on the project download page)

the default template has both a file and image upload field. just make sure that if you want to upload a file, that you enter it in the File: field and not the Image: field. The Image: field is optional. If you do not upload an image along with the file you are uploading, the default 'noimage' image will be displayed. You can designate what the 'noimage' image is on the repository module's settings page.