I've had the same problem when trying to upload .ipa files. The problem is, that .ipa files don't contain the symbolication information anymore.
Try exporting your app as Xcode Archive instead of building the .ipa for distribution (thats one of the options when you choose 'Distribute...' in the Xcode organizer). This archive contains the necessary dSYM files (if not, you have to check your build settings). If you drop this .xcarchive into the TestFlight desktop client, TestFlight will build the .ipa for you and the upload it.
Make sure to use the same Provisioning Profiles for signing the .ipa in the TestFlight client that you have used when building the archive in Xcode!
Or you can just choose to Show the TestFlight Co-pilot in the menu bar (activate in the settings), the it will automatically detect your Archive.