Fixing the base64 rule

[gregoirelinot]

Fixing the regex as base64 can start with something else than [a-zA-Z-0-9], a false assumption which I took when creating the regex initially. An example of this is the first bytes of a jpeg images: FF D8 FF which is /9j/ in base64

[macbre]

Thanks for this PR. Would be great if you can add a test case with base -64 encoded jpeg that you've mentioned.

[gregoirelinot]

Thanks for this PR. Would be great if you can add a test case with base -64 encoded jpeg that you've mentioned.

will do