Make file format validation easier & add proper documentation

[keichan34]

• Use the file command to identify MIME type if available
• Add documentation

[keichan34]

@asiniy Go ahead! I was thinking about making a new processor and packaging it in exfile (and not exfile-imagemagick). Send a PR and I'll review it. If you aren't sure about it but you want me to look at it anyways, just send the PR and tell me not to merge immediately.

[asiniy]

@keichan34 thanks! I think this functionality will take more than half of the week anyway, because I need to understand essentials of the project

[asiniy]

@keichan34 close it?