The correct way to name the file is "Offene_Posten.csv". "Offenes_Posten.csv" is plainly false.
Explanation: In German adjectives need to match noun in number, gender and case. For your example the base-noun is "der Posten" (the item or asset, but also job position). It does not become clear if you have only one open item, or (more likely) a list of open items. Let's assume that the number is actual multiple ones (plural). The gender of "der Posten" is masculine. Lastly you need to identify the case. Since you are not using the noun in a sentence, the case will be the first case (nominative).
If you use google to search for a declination table for "offen" (base form of "offene") and look for plural, masculine, nominative you will probably see the suggestion that leads to a filename of "Die_offenen_Posten.csv". But since you are not including the article in your filename you have to look at the article less table, leaving you with the filename "Offene_Posten.csv".
Bonus: If your file truly only contains a single open item, then the filename could also be "Offener_Posten.csv" (same reasoning as above but for the combination: singular, male, nominative, without article).