I am aware of the difference between content and contents and usually the distinction is clear to me. However I am wondering whether file content or file contents is more correct. On the one hand, the content of a file is a finite sequence of bytes, hence contents would be appropriate. On the other hand, those bytes represent some entity (an image, for example), to which I would refer as the file's content. Is this distinction correct, or am I complicating things too much?
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.
The general usage is File contents (when referring to something contained)
Google Ngram also shows that File contents has an edge over file content
It seems more appropriate to use file contents.
Additionally, note that
maxlen parameters, thus tending to treat the file contents as a "sequence of bytes".
file_get_contents() returns the file in a string, starting at the specified offset up to maxlen bytes