OS? Version of AE? Type of tiff? Will any type of image sequence import as footage? I am not seeing this problem on my system. I work with image sequences almost every day.
I have tried to import Camera Raw files, TIFFs (normal .tif files created in PS from Camera Raw files), and JPGs. In each case, as soon as I select any one of these files, "Footage" goes away as an Import As choice, although AE recognizes the Format as TIFF, and allows me to check TIFF Sequence.
The Import As control is behaving as expected. It only makes itself available for certain file types. For example, if you import a layered PSD file, you can choose to import the file as a composition so each of the PSD layers becomes a separate footage item in AE.
The problem you appear to be having is that your TIFF or Camera Raw sequence is not importing as expected. Often this is due to your sequence files not being numbered consistently. It will help if you post a screenshot of the files you are trying to import, and to provide more detail about the workflow you are using.
Enable the Force Alphabetical Order option.
Your CR2 sequence clearly shows files missing in the numbering sequence. That is why you're getting this warning message. If your folder contains all the files that you want to use, then you either need to renumber the files or just use the Force Alphabetical option.
I see no numbering problems in your TIFF or JPEG sequences, but I can only see the one page you provided. If you're getting the warning message with those two sequences, the same logic applies. Do your choice of: go back to your source to find the missing frames OR enable Force Alphabetical OR correct the file numbering.
Well, interesting! I changed two variables - never helpful - but ONE of them did the trick. I changed the naming to include no extraneous numbers, for example, from "Brescia_TL1034" to simply "Brescia034". And I selected Force Alphabetical.
Now, the Footage option remained grayed-out, for some reason, but the frames did import as a sequence, and function normally as such.