"We don't support the file type you tried to import" keeps showing when I try to upload my data

Probably the file did not save completely, and is corrupted. Try repairing it with the Section Remover of FIT File Tools (just pass it through it, let it fix errors, and do not remove anything). Hopefully at least some of the data can be restored.

Thanks! How do I just let it pass through? I only see options to remove things from the file. 

Unknown said:

How do I just let it pass through?

Step 1: drop the file there » Step 4: Go

If you do not get any download link afterwards, rather just an error message, you may need to have the file repaired by an expert (if possible at all). Try posting the file to the sticky thread for FIT files at the top of this forum - developers there are usually very helpful.