[SalesForce] Export Visualforce Page in Excel as .xlsx and not as a Web Page.

Using a contentType of "application/x-unknown" and specifying the desired filename via setting the Content-Disposition header in the controller was key to making this work.

The working solution was the following code:

Controller

public with sharing class SAPGUILauncher {

    public SAPGUILauncher() {
        Apexpages.currentPage().getHeaders().put('Content-Disposition', 'attachment; filename="SAPGUILauncher.sap"');
    }
}

VF

<apex:page controller="SAPGUILauncher" cache="true" contentType="application/x-unknown">[System]
Name={!$Setup.SAPLauncherSettings__c.SAPName__c}
Description={!$Setup.SAPLauncherSettings__c.SAPDescription__c}
Client={!$Setup.SAPLauncherSettings__c.SAPClient__c}
[User]
Language={!$Setup.SAPLauncherSettings__c.SAPLanguage__c}
[Function]
Title={!$Setup.SAPLauncherSettings__c.SAPTitle__c}
Command=ZSFDCVA01_2 P_AUART={!$CurrentPage.Parameters.OrderType}; P_VKORG={!$CurrentPage.Parameters.SalesOrg}; P_VTWEG={!$CurrentPage.Parameters.DistributionChannel}; P_KUNNR={!$CurrentPage.Parameters.SoldToParty}; P_KUNWE={!$CurrentPage.Parameters.ShipToParty}; P_BSTKD={!$CurrentPage.Parameters.PONumber}; P_MATNR={!$CurrentPage.Parameters.MaterialNumber}; P_KWMENG={!$CurrentPage.Parameters.Quantity};
[Options]
Reuse={!$Setup.SAPLauncherSettings__c.SAPReuse__c}
</apex:page>

While the documentation in How to export data in Excel or PDF format using Visualforce says to create an Excel file from a Visualforce page all you need to do is set the contentType to application/vnd.ms-excel, it isn't really telling the whole story.

That will indeed prompt the browser to treat the page content as an Excel spreadsheet.

However, the content is actually still HTML (Excel Web Page support). If you save the results and open it in the text editor of your choice you will see that it starts with a <script> tag.

An actual xlsx file is actually a zip file, so it would start with the zip PK header.

Excel on the desktop is tolerant enough (with the prompt) to read the HTML in as a spreadsheet.

The mobile version your using doesn't appear to support this.

I'd suggest exporting as a CSV in this case, as the support will be more universal.

As you commented. The Knowledge Article Content type XLS not supported in Mobile browsers covers the same points.