Answer on this question is given here: https://forum.mendix.com/link/questions/97776
For getting and testing a regex there are great online tools available. For instance www.regex101.com and www.freeformatter.com
This regex will match you the file-extensions:
The AppStore widget ‘File dropper’ has a setting that limits the imported files to only the file-extensions you specify:
After reviewing my answer given on https://forum.mendix.com/link/questions/97776, I found a more secure RegEx that checks the file format better:
My previous regular expression only checked the file format value, and not if it was an actual file format extension. For example, with the old RegEx ‘.*(txt$|jpeg$|png$|pdf$|jpg$)’ both 'filename.txt’ and 'filenametxt’ would match the RegEx. With the new RegEx, only 'filename.txt’ would match.