Don't override content-type for blobs

[lukewarlow]

This PR fixes an issue where a File body would have its content type overridden with application/json.

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[lukewarlow]

@googlebot I consent.

[lukewarlow]

Let me know if there's anything else I have to do to make the CLA happy to get this merged

[developit]

Size Change: +65 B (2%)

Total Size: 3.02 kB

Filename	Size	Change
dist/redaxios.js	993 B	+21 B (2%)
dist/redaxios.module.js	994 B	+23 B (2%)
dist/redaxios.umd.js	1.04 kB	+21 B (2%)