"file content" vs. "file contents"

Solution 1:

You are correct. "Content" (without the S) implies there's some kind of meaning; not just a string of bytes. If you refer to thefreedictionary.com, the first definition that's NOT shown as synonymous with "contents" is "the substantive or meaningful part"; which in this case is the entity represented by the bytes.

Solution 2:

The general usage is File contents (when referring to something contained)

Google Ngram also shows that File contents has an edge over file content

Solution 3:

It seems more appropriate to use file contents.

For example, PHP programming language has two corresponding functions, named file_get_contents and file_put_contents.

Additionally, note that file_get_contents() accepts offset and maxlen parameters, thus tending to treat the file contents as a "sequence of bytes".

Taken from file_get_contents() documentation:

file_get_contents() returns the file in a string, starting at the specified offset up to maxlen bytes