Quote from xchellx on August 28, 2019, 10:30 am

The FileOpenBox method has a File mask: parameter. This parameter's syntax may be confusing at first, especially when the VisualNeoWin help topic explains it poorly.

This parameter will control what the selections Files of type: dropdown box in the bottom of the dialog box will have. This dropdown can be used to filter what is shown in the current directory to the dialog box.

The is an example to show how syntax goes as follows:
Text Based Document (*.txt, *.rtf)|*.txt;*.rtf|Text Document (*.txt)|*.txt|Rich Text Document (*.rtf)|*.rtf|Any File (*.*)|*.*

Which results in:
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Looking at the example, notice how this has parts.
The first part (file mask name) is the name that should appear in the dropdown. Like "This is a file filter test 1"; it can literally be anything.
The second part (file filter) is the file filter. The * represents wildcard which literally means anything. Unless you want to filter for a specific file with a specific file name, you should use a wildcard before the extension. This can literally be any extension, for example: *.pub. This will make it so only pub files will only appear out of all the other files in a folder that the dialog is viewing.

The vertical bar (|). is used to separate the file mask name, and the actual file filter the name represents (the first and second part, respectively). The vertical bar (|) is also used to separate one file mask set from another (so you can use more than one file mask in the dropdown. The example used above will show Text Based Document (*.txt, *.rtf), Text Document (*.txt), Rich Text Document (*.rtf), and Any File (*.*) as selections in that dropdown.).
Semi-colon (;) is used to separate two or more file filters if you want to match multiple types of files via one file mask. For the above example, *.txt;*.rtf will match both txt and rtf files when the file mask Text Based Document (*.txt, *.rtf) is selected via the file open dropdown.

Note: You can use *.* to match everything anything (don't filter). Try to experiment with this. An example would be Custom File Mask 1 (testfile_*.txt)|testfile_*.txtwhich will only show files that are named testfile_(anything)with the extension of txt. For this example, testfile_1.txt would be shown but testfile1.txt would not be shown.

Bonus: Here is a filter I made for HTML and HTM, HTML, HTM, or Any File:
Hypertext Markup Language (*.HTML, *.HTM)|*html;*.htm|Hypertext Markup Language - Proper Extension (*.HTML)|*.html|Hypertext Markup Language - Improper Extension (*.HTM)|*.htm|Any File (*.*)|*.*