I will move your request to the Qlik Sense - Integration, Extensions and APIs - section. Hopefully it will get some visibility. If you do not hear anything, please let us know. I will be notified via email and will attempt to get someone that can assist.
I do not believe uploading a file is part of the SDKs, the Folder data connection will work with files that are on local drives, UNC, network share and mapped drives. A WebFile connection will work with all file-types that we support with the Folder Connection but over HTTP. I would assume you would implement the code that enables a UI to select an .xls file and then upload it to an appropriate path that the Qlik Sense server would read. Let's see what others may say, though.
In the mean-time, have you looked at the Developer Help Site - to see if what you are looking for is there:
THIS IS JUST A THOUGHT - BUT MAY NOT BE recommended, depending on the size of the file - YOU TECHNICALLY - could UPLOAD a FILE to the content repository and then reference the file that way. I believe the content repository data is also URL addressable. You could use that URL for the xls file in your web file data connection. However, depending on the size of the files, this may not be an approach you are looking for. I am not sure if it actually stores the files as a BLOB, or if it places the file on disks in a temp location and then stores the index and metadata about the file in the repository database.
There is a POST method that could technically do this.