That seems similar to this known issue:
For More Information see: WebDirect Export Field Contents Inconsistent with FileMaker Pro
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Hmmm. I tried to use the .xls extension to try to get a .xml extension (which should still default to open in a browser). I get no extension regardless of what extension I put on the source file.
I suggest posting an Issue Report to see what the FileMaker Techs have to suggest on this. This may be a different bug, a related bug or there is some setting that I don't know about that will make this work for you.