"file content" vs. "file contents"

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

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

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.

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