Added hadling of allowed_extensions for file uploads in backend's configuration section

[eneiasramos]

Added support for tag allowed_extensions in file upload in backend system configuration.

[kiatng]

@eneiasramos It will help to review if you can provide the use case.

[addison74]

@eneiasramos - Please provide more details about this PR. I have some time to test it.

[eneiasramos]

@kiatng @addison74 my dears

The routine here filters the extension of a file being uploaded.

So if I upload a file with an extension different from .pfx, the system will display an error.

A use of case: I use the var folder to store PFX certificates used to communicate with a third party system:

https://github.com/gamuzatech/tolucastore-openmage/blob/f559a0e819a3124599b392fc3c4defe5c3803b68/app/code/local/Gamuza/Brazil/etc/system.xml#L400

[kiatng]

Somewhat related to PR #4079, extend functionalities of file upload in system config.

[kiatng]

LGTM. I like it.

[fballiano]

tested, working, thank you!

[eneiasramos]

@fballiano It's my pleasure. :)