Add mimetype check for XML files

[skalupa]

Describe the change
Adds in a mimetype check to verify that the given file is in fact an XML file when an error is thrown by the XML scanner in order to verify that the file in question is a XML file. This is done in the error handling section of the scanner instead of the start of the scanner in order to preserve the validity of the existing test cases.

Describe testing procedures
Tested with opensource Strelka in order to see results in the results in the StrelkaUI, which will be shown below. Specifically tested with .zip files in order to emulate cases in which this was seen in other worker instances

Sample output
With the above change, the ScanXML bucket still appears in the UI, though contains only the following flag:

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings