allows to open the most common extensions for markdown

[cfgnunes]

This "pull request" allows to open the most common extensions for Markdown files.

If the files contain the following extensions:

• .md
• .markdown"
• .txt the application shows them in the list (not only ".md" extension).

This fix/feature is important because it makes it possible to open files that already exist in the directory.

Furthermore, we don't always want to write notes in Markdown format: sometimes we just want to write a plain text file (.txt). 😊