Learn English – “Username”, “user name” or “user-name”

“Computer related” is a compound adjective and whether or not you should use a hyphen depends on where it is located in the sentence. If it appears before the word it modifies, include a hyphen. If after, omit the hyphen. This is mostly a clarification of Ex-user's answer. To use his/her examples, the following are both correct:

1. This book is computer related. (the noun being modified appears before the compound adjective)
2. This is a computer-related book. (the noun being modified appears after the compound adjective)

Use of hyphenated compound adjectives often prevents ambiguity of meaning (here's a good example), but when there can only be one meaning, hyphens may be omitted. I usually hyphenate anyway to be safe, and because it's one less thing to consider when writing. Additional usage information.

1. I think that the second line is a better explanation in the context shown, but there are plenty of other places where the simpler 'filename' (or 'file name') would be used in documentation without problem.

2. Yes, using the articles as shown is correct.

3. Compound sequences of nouns like 'application executable file name' are used quite a lot, but often don't make for good reading. On the other hand, neither does the form using 'of the' if it is repeated often. That's why people tend to end up using a shorthand - 'application' or 'application executable' for the example, depending on exactly what is being discussed.