Assistant, when QTextBrowser opens link to plain text file, it parses the file as it is rich text format

When opening a link to a local text file in assistant, the file will be parsed as it is rich text file.

• the blank space/return will be shown incorrectly (in html, blanks will be neglected.)
• if the file contains some special chars in html, then the file might be parsed and shown truncated.(for example, the file which contains "<", text after "<<<" is truncated.)