Add support for other text formats in the file content filter

[dpomykala]

Change Summary

The filecontent filter now uses a specialized extractor function if registered for the given format (currently PDF and DOCX). For all other formats the extract_txt function is used as a fallback.

This allows to use the filecontent filter for all plain-text files, regardless of the file extension.

Related issue number

Resolves #464.

Checklist

• [x] Tests for the changes exist and pass on CI
• [x] Documentation reflects the changes where applicable
• [x] Change is documented in CHANGELOG.md (if applicable)
• [x] My PR is ready to review